The alligator and its allies

by A. M. Reese

The Histology of the Enteron of the Florida Alligator

It has long been known that the sea lamprey, Petromyzon marinus, during the spawning season, when the body is distended with eggs, takes no food, and that the digestive tract during this period shrivels up until it is reduced to a mere thread. This condition doubtless obtains in other forms as well, though it has not been actually observed by the writer elsewhere.

A number of small alligators that were kept alive in the laboratory for a year or more caused the writer to wonder whether any very marked change had taken place in their digestive tracts during the months they took no food.

In captivity, especially if the water in their tank be kept cold, alligators may refuse food for five or sixth months. Whether, during the winter months, in their native haunts, they entirely cease feeding, the writer has had no opportunity to observe, though it is popularly reported that such is the case.

The first alligator from which tissues were taken was about a year and a half old, and measured eighteen inches in length. It was killed in March after a fast of several months, probably four or five, possibly more, though it was not in the writer’s possession for so long a time.

Fig. 35. Outline of Digestive Tract

Fig. 35. A diagrammatic outline of the digestive tract of the alligator from the beginning of the œsophagus to the cloaca, to show the planes of the sections that were studied, a.oes., anterior œsophagus; a.r., anterior rectum; a.s.i., anterior small intestine; c.st., cardiac stomach; f.st., fundic stomach; m.s.i., middle small intestine; p.oes., posterior œsophagus; p.r., posterior rectum or cloaca; p.s.i., posterior small intestine; p.st., pyloric stomach.

Although carefully fixed in the usual fluids, the epithelial structures from this animal were not as clearly defined in most cases as could be desired; this rather unsatisfactory fixation may have been due to some physiological condition characteristic of the period of hibernation. That this was the case seems likely from the better fixation obtained by the same methods in the case of animals killed during the feeding season.

The other animals from which tissues were taken were considerably smaller than the one mentioned above. They were killed early in the fall, after having been fed regularly for about five months upon bits of meat, both raw and cooked.

The Tongue.

The covering of the tongue was studied in two regions, near the free end, and towards the base.

A section of the former region, drawn under high power, is shown in [Figure 36]. It consists of a dense mass of fibrous tissue, a, and small scattered cells, overlaid by a stratified epithelium of eight or ten layers. Only a small part of the fibrous base, just beneath the epithelium, is here shown. It is a dense areolar tissue with the elastic fibers apparently predominating.

The epithelium, e, consists, as has just been said, of about eight or ten layers of cells, those at the base being generally cuboidal in shape, while towards the surface the cells become more and more flattened until at the surface they form a thick horny layer, h, in which no nuclei can be seen. The cells of the horny layer are flattened into mere fibers, which, at places, are seen projecting from the surface. The boundary between the horny cells and those beneath is quite distinct, though perhaps not quite so sharp as shown in the figure under discussion.

Fig. 36. The covering of the anterior region of the tongue of the hibernating animal, under fairly high magnification; the plane of this section is not shown in [Figure 35]; a, areolar tissue; e, epithelium; h, horny layer of epithelium.

In a previous paper, the writer noted that the dorsum of the tongue is covered with small, evenly distributed papillæ, easily seen by aid of a hand lens. These so-called papillæ are here seen to be hardly papillæ at all, but small folds or wrinkles, although the epithelium is somewhat thickened at intervals. No glands are to be seen in this region of the tongue.

The only difference between the anterior region of the tongue during hibernation and during the feeding season seems to be in the scaly layer of the epithelium. Instead of the compact, sharply differentiated layer of scaly cells seen in [Figure 36], the anterior region of the tongue during feeding is covered with a layer of rather loose, scaly cells, in most of which the nuclei may be seen. No difference in the amount of sloughing off can be noticed as is the case with the epithelium of the roof of the mouth.

[Figure 37] represents a section, under very low magnification, of the covering of the base of the tongue. The areolar tissue, a, is about the same as in the preceding section, except that it is more compact just under the epithelium than it is in its deeper regions. It seems also more vascular than in the preceding section.

Fig. 37. Covering of the posterior region of the tongue of the hibernating animal showing glands, under low magnification; a, areolar tissue; bv, blood-vessels; g, glands; e, epithelium.

The epithelium, e, is of the stratified squamous variety, but consists of many more layers of cells than in the preceding section and is hence several times as thick. While its cells are flattened towards the surface, after the manner of this kind of epithelium, they do not form the definite horny layer described above.

Fig. 38. One of the glands from the posterior region of the tongue of the hibernating animal, under high magnification; a, areolar tissue; av, alveolus.

The most marked difference between the two regions of the tongue is the presence, in the posterior or basal region, of numerous glands, g, probably mucous- or slime-secreting. They are thickly scattered through the areolar base, close beneath the epithelium. Two large glands and one small one are shown in the figure under discussion. Each gland opens to the surface by an apparently wide duct, but since no good section of such a duct was obtained it is not shown in the figure. Although the rest of the tissue was well preserved and showed cell structure clearly, it was with difficulty that the details of the glands could be determined.

A high-power drawing of a portion of one of the glands is shown in [Figure 38]. The large alveolus, av, to the left, is from the peripheral region of the gland and is surrounded, on its free side, by the areolar tissue described above. The inter-alveolar spaces, which are somewhat exaggerated in the figure, are filled with fibers which are arranged more or less in layers and hence appears different from the surrounding areolar tissue. The alveoli are circular or elongated in section, and have fairly wide lumina. They are lined with a single layer of columnar or cuboidal cells which are very granular, so that their walls are difficult to determine. Each cell contains, near its base, a very large, usually spherical nucleus. These nuclei stain darkly and give the dark appearance to the glands as seen under low magnification, especially in rather thick sections.

During feeding the epithelium of this region of the tongue consists of fewer layers of cells than during hibernation but is otherwise unchanged from what is described above. The glands consist, at least in all of the material examined, of much fewer alveoli than are shown in [Figure 37]. One of these glands is shown in [Figure 39].

Although no more care was used in fixation than in the corresponding tissue of the hibernating animal the glands here show their cell details far more clearly than in the former tissue; this may have been partly due to the latter sections being thinner.

The glands are of a compound, tubulo-alveolar type; although numerous sections through ducts were obtained (as in [Fig. 39]), no details of these ducts could be seen.

Fig. 39. One of the glands from the posterior region of the tongue of the feeding animal, under somewhat higher magnification than used in [Figure 37]; av, alveolus; d, duct of gland; e, stratified epithelium.

As noted above, and as may be seen by comparing [Figures 37] and [39], the gland during hibernation, at least in the animals studied, consists of many more alveoli than during the feeding season; this, of course, might not prove to be always the case if larger numbers of animals were studied; the difference in the ages of the animals might have caused this difference in the glands. In the material studied the largest glands from the hibernating animals consist of more than twice as many alveoli as the glands in the feeding animals. As seen under high magnification there is no noticeable difference in the glands at the two seasons.

Rathke has given the name of “Geschmackwärzchen” to the conical projections found on the dorsum of the crocodilian tongue; they are distinguished by their softness and thinner epithelial covering from the cones that, in many of these animals, bear the openings of the mucous glands.

These taste papillæ generally have the form of a truncated cone and often are surrounded by a shallow circular pit, outside of which, in turn, is sometimes a small low wall. They are distributed over the entire dorsum of the tongue, usually at considerable distance from each other in comparison to the size of the tongue, and are not so numerous as the taste papillæ of the Mammalia. Rathke found their absolute number greatest in A. lucius.

Rathke mentions other larger and harder projections on the tongue of certain Crocodilia which, though not perforated by a mucous duct, he thinks are of questionable relation to the sense papillæ. They usually have more the form of a flattened than of a truncated cone, and are very numerous in some species.

The Roof of the Mouth.

In the paper mentioned above the author notes that the papillæ on the roof of the mouth are evenly distributed and are more distinct than those of the dorsum of the tongue. One of these papillæ as seen under fairly high magnification is shown in [Figure 40].

Fig. 40. The covering of the roof of the mouth of the hibernating animal, under fairly high magnification; a, areolar tissue; e, epithelium; h, horny layer; f, fibers of horny layer.

The areolar tissue, a, forming the base of the section is of about the same character as seen in the section of the tongue. Less than one tenth of the thickness of the entire areolar base is shown in this section.

The epithelium, e, where not thrown into papillæ, has also about the same character as that of the anterior region of the tongue—the same number of cell layers and the same distinct horny layer.

At intervals the thickness of the cellular part of the epithelium is greatly increased, and at the same time the horny layer is also thickened, to form distinct papillæ like the one shown in the figure. These, as has been said, are comparatively small and have the shape of a blunt cone. The center of the cone is, of course, made up of the cellular epithelium, while the outside is covered with the thickened horny layer from which fibers, f, are often seen projecting. Near the apex of the cone the nuclei are larger and more widely scattered than those at the base.

No glands were seen in the roof of the mouth of the hibernating animal, but since the entire roof was not sectioned it is probable that they may exist in some regions; in fact, as noted below, sections through the posterior region of the roof of the mouth of the feeding animal do show numerous glands.

As might be expected there is comparatively little difference between this region of the enteron during hibernation and during the feeding season. The only noticeable difference is in the stratified epithelium; that of the feeding animal not only has less sharp papillæ but has also a much thinner scaly layer of cells. As is seen in the [figure] of the roof of the mouth during hibernation the scaly cells make up, except on the papillæ, nearly or quite half of the thickness of the epithelium, while in the feeding animal they make up not more than one fourth or one third of the entire epithelium. Very few cells are seen sloughing off as in [Figure 40]; possibly the act of feeding keeps the superficial scaly cells rubbed off smooth.

In the extreme posterior region of the roof of the mouth the epithelium consists of a greater number of layers (though the number is very variable) than in the region shown in [Figure 40]. In this posterior region, as noted above, glands are found. These glands have the same structure as those described in connection with the posterior region of the tongue.

The Œsophagus.

Sections of the œsophagus were made from two regions, an anterior, half-inch caudad to the pharynx, and a posterior region, half-inch cephalad to the opening of the œsophagus into the stomach ([Fig. 35]).

The general structure of the wall of the œsophagus, as seen under a low magnification, will first be described, after which the minute structure of the epithelium, as seen under high magnification, will be discussed.

In the anterior region the usual layers of the vertebrate enteron are present, except, possibly, the muscularis mucosa.

The epithelium, to be described later, is, together with the submucosa, thrown into complicated folds; its closely arranged and darkly stained nuclei cause it to stand out in strong contrast to the other tissues of the section ([Fig. 41], e).

The submucosa, sm, is of considerable thickness. It is composed of a fairly dense mass of connective tissue, mainly elastic fibers, through which are scattered small blood-vessels, bv, and small dark areas, mb, that are apparently longitudinal bundles of involuntary muscle fibers. These few and scattered fibers probably represent the muscularis mucosa that is so well developed in the posterior region of the œsophagus.

Outside of the mucosa is a thick circular layer of involuntary muscle fibers, cm, the fibers being collected into irregular bundles, between which are narrow spaces filled with connective tissue that contains a few small blood-vessels.

Surrounding the circular layer is a thinner and less clearly defined layer of longitudinal muscle fibers, lm. The muscle bundles are more definite than in the circular layer and are separated from each other by a considerable amount of connective tissue with a few small blood-vessels.

Fig. 41. A transsection through the anterior region of the œsophagus of the hibernating animal under low magnification; bv, blood-vessels; mb, muscle bundles; other letters as in [Figure 42].

The serosa, s, is here quite indistinct. It consists of a slightly vascular connective tissue which cannot be distinctly differentiated from the connective tissue of the longitudinal layer.

Fig. 42. A transsection through the posterior region of the œsophagus of the hibernating animal, under low magnification; e, epithelium; cm, circular muscles; lm, longitudinal muscles; mm, muscularis mucosa; sm, submucosa; s, serosa.

In the posterior region of the œsophagus, as may be seen by comparison of figures 41 and 42, the wall as a whole is about one third thicker than in the anterior region just described, though how much of this difference is due to different degrees of distension or contraction it is hard to say.

The epithelium, e, is in the tissue studied thrown into less complicated folds than in the anterior region, and is not so thick.

The submucosa, sm, if the entire layer may be so called, has about the same thickness and structure as in the more anterior region; but instead of the small and widely scattered bundles of longitudinal muscle fibers there is a distinct layer of muscle which may be called the muscularis mucosa, mm, lying about midway between the epithelium and the circular muscle layer.

The muscularis mucosa is somewhat variable in thickness and is thrown into folds that correspond to the larger folds of the epithelium and the submucosa; one of these folds is shown in [Figure 42]. The fibers of the muscularis mucosa are apparently all longitudinal in position.

Outside of the submucosa is a layer of circular muscle fibers, cm; it is here somewhat wider and more dense than in the anterior region.

The longitudinal muscle layer ([Fig. 42], lm) is much wider and more compact than in the anterior region. The fibers are indistinctly divided into large irregular masses as shown in the figure.

The serosa ([Fig. 42], s) is a varying but fairly thick layer that is quite distinct from the longitudinal muscle layer. It consists of the usual connective tissue groundwork with scattered blood-vessels.

The epithelium, as was said above, is thicker and somewhat more folded in the anterior than in the posterior region, and in the former region is partially ciliated while in the latter cilia are entirely wanting. With these exceptions the epithelium is practically the same in the two regions.

[Figure 43] represents the epithelium from the anterior region as seen under high magnification. The outlines of all the cells could not be determined but if each nucleus represents a cell there are twenty-five or thirty layers of cells. The nuclei are arranged in two dense, irregular groups, one along the base of the epithelium, the other about two thirds of the distance from the base to the free border. The basal nuclei are perhaps slightly larger and more rounded than those of the distal group. Between these two groups are numerous more scattered nuclei; while scattered through the epithelium, except near the free border, are smaller, round nuclei that stain somewhat darker than the rest; these, from their size and appearance, seem possibly to belong to an invisible network of connective tissue that has penetrated the epithelium from the surrounding mucosa.

Fig. 43. The epithelium of the anterior region of the œsophagus of the hibernating animal, under high magnification.

The free border of the epithelium consists of long, ciliated, columnar cells in which the cell walls may be easily seen. The cilia are of average length and even in this anterior region are not everywhere present; possibly they are arranged in bands, but the material at hand was not sufficient to determine this. As was noted above, cilia are wanting in the posterior region.

The only differences noted in the anterior region of the œsophagus between the feeding and the hibernating conditions are in the muscularis mucosa and the epithelium. As was noted above, the muscularis mucosa is practically absent in the hibernating stage, being represented only by a few small, scattered bundles of longitudinal muscle fibers; while in the feeding stage there is a narrow but fairly distinct layer to represent the muscularis mucosa.

Fig. 44. The epithelium of the anterior region of the œsophagus of the feeding animal, under high magnification.

The difference in the appearance of the epithelium is not striking. The nuclei are somewhat larger in the feeding stage and, instead of being crowded into a basal and a median zone, as noted in the hibernating conditions, they form a dense basal zone, but show no indication of medial zone. From the dense basal zone the nuclei become more scattered towards the free surface and are rarely found closer to the surface than is shown in [Figure 44]. The smaller nuclei scattered among the larger ones, noted in connection with the hibernating stage, are not here seen.

As in the hibernating stage cilia are present on some but not all cells of this region.

The only noticeable difference between the feeding and hibernating conditions of the posterior region of the œsophagus is in the epithelium, which, as in the feeding condition of the anterior œsophagus, exhibits but one zone of closely set nuclei, that at the base of the epithelium.

The Stomach.

The stomach was sectioned in three regions, as shown in [Figure 35]: (1) in the cardiac region very near the opening of the œsophagus; (2) in the middle or fundic region; and (3) in the region near the opening of the pylorus. The first two sections are in the first or large region of the stomach; the third section is in the second or small region of the stomach ([Fig. 35]).

The wall as a whole is thickest in the fundus, being there practically twice as thick as in the pyloric and half again as thick as in the cardiac region. This great thickening is due mainly to a thickening of the middle or oblique layer of muscle, which is here remarkably developed. The mucosa is of nearly uniform thickness in the different regions and will be described later.

Since there is no striking difference beside that of thickness in the general structure of the wall of the different regions, the pyloric region, as seen under low magnification, will now be described ([Fig. 45]).

The mucosa, m, consists of fairly long glands underlaid by a well-marked muscularis mucosa, mm, the latter exhibiting a compact circular layer over a wider but more scattered layer of longitudinal fibers. A considerable amount of fibrous connective tissue lies among the muscle fibers. The circular layer of the muscularis mucosa sends towards the surface numerous strands or septa between the glands; six or eight of these are seen in the figure. These strands are not nearly so numerous in the large region of the stomach. As was said, the outer or longitudinal layer of the muscularis mucosa is wider but less compact than the circular and its bundles of fibers are seen in the [figure] as a layer of large, scattered dots just beneath the circular layer.

The submucosa, sm, is of average thickness and density. In the fundic and cardiac regions it seems to extend between the circular and oblique layers; at any rate, there is a considerable layer of connective tissue between these two muscular layers.

The circular muscular layer, cm, is of only moderate thickness and is of rather a loose character. In the pyloric region it is not very distinct from the underlying oblique layer, but in the other regions, as has just been said, it is separated from the oblique layer by a considerable layer of connective tissue like that of the submucosa.

The oblique layer, om, even in this section of the pyloric region is the thickest of the three muscle layers; while in the cardiac, and especially in the fundic, regions it is of great thickness, as was noted above, and is made up of larger bundles with less intervening connective tissue.

Fig. 45. A transsection through the wall of the pyloric region of the stomach of the feeding animal, under low magnification; m, mucosa; om, oblique muscles; other letters as in [Figure 42].

The outer or longitudinal muscle layer, lm, is comparatively little developed and consists of small rather scattered bundles of muscles with a correspondingly large amount of connective tissue. This connective tissue passes insensibly into that of the surrounding serosa, s, a loose, vascular layer of varying thickness and density, shown very thick in [Figure 45], but often much thinner.

So far as could be determined, the mucous membrane has the same structure in both anterior and middle regions of the stomach. That of the pyloric or small region, although fixed, stained, et cetera, just as carefully as the rest, did not show cell details sufficiently well to draw; the ducts of the glands in this region are fairly distinct but the deeper parts of the glands have the appearance of series of alveoli or large adipose cells. What the significance of this condition may be the writer is not able to say, but since the structure of this region of the gastric mucous membrane is not clear no attempt will be made to describe its appearance under higher magnification than was employed in the [figure] above. However, as will be noted below, there is probably no great difference between the pyloric mucosa and that of the other regions of the stomach.

Fig. 46. The glands of the middle or fundic region of the stomach of the hibernating animal, under high magnification; A, through duct; B, through body of gland; C, through fundus of gland.

[Figure 46] shows portions of typical glands from the mucosa of the middle region of the stomach, the posterior border of the large stomach cavity; A is a longitudinal section through two ducts where they open to the surface; B is a similar section through the body of a gland below the region of the duct; C is a transsection through the bottom or fundus of a gland; all are drawn with a camera under the same magnification.

As is seen in [Figure 45], under low magnification, the duct is about one third of the entire length of the gland. The lumen of the duct is fairly wide, that of the body of the gland is reduced to a mere slit, while that of the fundus is quite wide.

One, two, or possibly more, glands may open to the surface through one duct, as is shown in [Figure 46]. There is nothing peculiar about the epithelium of these glands. Near the opening of the duct the cells are of a typical columnar character with finely granular cytoplasm, each with a nucleus at its basal end.

In the deeper parts of the duct the cells become shorter until in the body of the gland ([Fig. 46], B) they are cuboidal in outline.

The bodies of the glands are so closely packed together that it is difficult to pick out an individual tube that will show details clearly enough to draw with a camera lucida. So far as could be observed all of the cells of this region of the gland are alike.

The bottom or fundus of the gland, as seen in [Figure 46], C, is somewhat enlarged and has a wide lumen. The cells are of the same general character as in the more distal parts of the gland except that they are somewhat more columnar or pyramidal than in the body of the gland. The nuclei of the body and fundus are usually somewhat larger and more nearly spherical than in the columnar cells of the duct.

The feeding animals from which tissues were taken were considerably smaller than the hibernating specimen, so that the stomach walls were proportionately thinner; but, so far as could be discovered, there was no difference in structure.

The relative thickness of the entire wall in each of the three regions sectioned was about the same as described above.

As has been said, the mucosa on the pyloric or small region of the stomach from the hibernating animal was so poorly fixed that its structure could not be made out. In the feeding stage the mucosa of this region was as well fixed as any of the other tissues and showed that its structure is essentially like that shown in [Figure 46], except that the glands are proportionately not quite so long as in the fundic and cardiac regions, and are somewhat more open—that is, they have wider lumina; their lining cells are all of one kind and are unchanged from what was seen in the hibernating condition.

The Small Intestine.

Three regions of the small intestine will be described: (1) an anterior, just caudad to the stomach; (2) a middle; and (3) a posterior, one half inch cephalad to the rectum or large intestine ([Fig. 35]).

As might be expected, the general structure of the wall of the intestine is essentially the same in all three regions, the slight differences noticeable being due mainly to variations in the thickness of the various layers.

The middle and posterior regions have about the same diameter, while the diameter of the anterior region is considerably greater, due partly to the greater diameter of the lumen but mainly to the greater thickness of the constituent layers, especially the mucosa. The mucosa is also thrown into more numerous and complicated folds in the anterior than in the middle and posterior regions; the complexity of the mucosa seems to diminish as the intestine is followed caudad. In the anterior region the mucosa may form at least one half of the entire thickness of the wall, while in the posterior region it may form less than one third of the thickness of the intestinal wall. The minute structure of the intestinal epithelium will be described below.

The chief peculiarity of the intestinal wall is the apparent total absence of a submucosa ([Fig. 47]). As will be described later, the mucosal epithelium is laid upon the usual bed of fibrous and lymphatic tissue, the tunica propria ([Fig. 47], tp).

At the outer border of the tunica propria, and with no tissue corresponding to a submucosa between it and the circular muscular layer, is a thin and indistinct layer that has the appearance of a longitudinal layer of muscle fibers; this should correspond to the muscularis mucosa ([Figs. 47], [48], [49], and [51], mm).

Fig. 47. A transsection of the wall of the anterior region of the small intestine of the hibernating animal, under low magnification; ln, lymph node; tp, tunica propria; other letters as in [Figure 42].

The circular, cm, and longitudinal, lm, muscle layers are compact, and are distinct from the other layers of the wall; the former is approximately twice the thickness of the latter. The relative thickness of all the layers in the three regions of the intestine may be seen by comparing [Figures 47], [48], and [49].

Fig. 48. An outline of a transsection of the wall of the middle region of the small intestine of the hibernating animal, under low magnification; lettering as in [Figure 42].

The serosa, s, which is of about the same character in the three regions under discussion, is a distinct and fairly dense layer of connective tissue with numerous blood-vessels.

The general appearance of the mucous membrane as a whole is sufficiently clear in the low-power drawing described above, so that all that need be shown under a higher magnification is the epithelium ([Fig. 50]). The upper part of this figure represents the lower end of one of the intestinal glands cut longitudinally, below which is the end of another gland in transverse section. Between the two sections is the compact tunica propria of lymphatic tissue.

The section from which this particular figure was drawn was in the anterior region, but the corresponding part of a section in either of the other regions would have practically the same appearance.

The epithelium is of the stratified columnar type. The superficial cells are very tall and narrow, with the nuclei generally at or near the bases, though an occasional nucleus may be seen near the free end of a cell. Below the tall columnar cells are four or five rows of nuclei which represent smaller, irregular cells, though the cell walls could not always be determined between the closely packed nuclei. No goblet cells are to be seen at any place.

Fig. 49. An outline of a transsection through the wall of the posterior region of the small intestine of the hibernating animal, under low magnification; lettering as in [Figure 42].

The relative diameters of the three regions of the small intestine in the feeding condition are about the same as noted for the hibernating stage; that is, the anterior region has the greatest diameter and the other regions are smaller and have about the same average diameter.

The most marked difference between the intestine during hibernation and feeding is in the relative thickness of the mucosa and muscular layers. As described for the hibernating stage, so in the feeding stage, the mucosa is relatively the thickest in the anterior regions and diminishes in thickness caudad; but while, in the hibernating stage, it forms, in the anterior region, as much as half of the entire thickness of the wall, in the feeding condition it forms, in the same region, at least two thirds of the entire wall and in the middle and posterior regions more than half of the wall.

Fig. 50. Part of the mucous membrane of the anterior region of the small intestine of the hibernating animal, under high magnification. The upper part of the figure shows a part of a gland cut longitudinally, the lower part of the figure shows another gland cut transversely; e, epithelium; tp, tunica propria.

The feeding animals being the smaller, the diameter of the intestine was considerably less than in the hibernating stage; but the actual thickness of the mucosa was practically the same, so that the difference in diameter was due to the difference in the thickness of the muscular and fibrous layers. It is therefore probable that the differences noted above are due rather to the differences in the size of the animals from which the tissues were taken than to the different conditions of hibernation and feeding. The point to be noticed is that the increase in the diameter of the intestine is due almost if not entirely to an increase in thickness of the connective tissue and muscle layers.

No difference in the complexity of the folds of the mucosa of the two stages can be noticed.

The thickness of the fibro-muscular part of the wall of the intestine varies considerably on different sides of the same region, but it consists of the same layers in about the same relative amounts.

Fig. 51. An outline of a transsection of the wall of the middle region of the small intestine of the feeding animal, under low magnification; m, mucosa; other letters as in [Figure 42].

[Figure 51] represents in outline the wall of the middle region of the small intestine during feeding.

The epithelium is of the same thickness in the two stages, and the only difference in its character that can be seen under a high magnification is that, in the middle region at least, the nuclei are not crowded so close together at the basal ends of the cells as in the hibernating stage but are scattered more towards their free ends.

Fig. 52. A transsection of the wall of the anterior region of the rectum or large intestine of the hibernating animal, under low magnification; tp, tunica propria; other letters as in [Figure 42].

Altogether, the differences in microscopic structure between the small intestine of an alligator at the end of the hibernating period and at the end of a period of regular feeding are very slight.

The Large Intestine.

The planes of the two sections studied are shown in [Figure 35]; a low-power drawing of the posterior region is shown in [Figure 52]. The anterior and posterior regions of the large intestine do not differ from each other sufficiently to make it worth while to represent both by drawings. Had an entire section through either region been drawn it would be seen that the wall is of very different thickness in different places, as was noted in connection with the small intestine; the posterior section was drawn where the wall was thin.

It might be supposed that in the feeding season the fecal matter in the posterior region of the rectum would stretch the walls sufficiently to obliterate largely the prominent folds seen in [Figure 52], but such does not seem to be the case. The usual layers of the vertebrate intestine are present.

Fig. 53. The epithelium of the anterior region of the rectum of the hibernating animal, under high magnification; e, epithelium; tp, tunica propria.

The epithelium, shown under high magnification in [Figure 53], is of the same character and thickness throughout, except that as the anal aperture is approached the columnar epithelium changes into the stratified variety. It consists of very tall and narrow columnar cells apparently in one layer, though it is difficult to be sure of this. With an occasional exception, near the top, all of the nuclei are arranged in a fairly wide zone below the middle of the epithelium. The nuclei are oval in shape and lie so close together that it is difficult, as has been said, to be sure that the cell to which each belongs extends throughout the entire thickness of the epithelium.

Beneath the epithelium ([Fig. 52], e) is a dense tunica propria, tp, underlaid, in turn, by the muscularis mucosa, mm, and a submucosa, sm, of the usual character, which is thrown into marked folds. The circular, cm, and longitudinal, lm, layers are of the usual character except that they vary more in thickness, as noted above, and in density than is usually the case.

The serosa, s, is comparatively thin and compact in both regions, and varies somewhat in thickness at different places.

The large intestine of the feeding animal was sectioned in the same regions as in the hibernating. As has been said, the feeding animals used were much smaller than the hibernating, so that, as might be expected, the diameter of the large intestine was much less in the former than in the latter. Except for this difference in diameter there was no noticeable difference between the two stages. In the case of the small intestine, it will be remembered, the greater diameter of the intestine of the larger animal was mainly due to the greater thickness of the muscular and connective-tissue layers and not to any increase in thickness of the mucous membrane. In the large intestine the mucosa varies in thickness in the animals of different size as do the other layers of the wall.

The glandular character of the lining of the large intestine seems to indicate that this region of the intestine must have some digestive or absorptive function and that it does not act merely as a receptacle for fecal matter; this makes it all the more strange that there should not be some change produced in its structure by five or six months of feeding or of fasting.

Summary.

The material used in this investigation was taken from young animals at the end of a feeding period of about five months, and towards the end of the hibernating period after fasting for four or five months.

The regions of the enteron that were studied were as follows: the tip and base of the tongue; the anterior and posterior regions of the roof of the mouth; the anterior and posterior regions of the œsophagus; the cardiac, fundic, and pyloric regions of the stomach; the anterior, middle, and posterior regions of the small intestine; the anterior and posterior regions of the large intestine. Since the work was started at the end of the hibernating period, the tissues of that period were studied and drawn first.

The only difference between the structure of the tip of the tongue during hibernation and during the feeding season is that the scaly epithelium with which it is covered is somewhat thicker and more compact in the former than in the latter condition, though even this difference may have been due to differences in the ages of the animals used. The base of the tongue differs from the tip in having a thicker epithelium and in having compound tubulo-alveolar glands. These glands in the hibernating animal have many more alveoli than in the feeding animal, though this, again, may have been due to the difference in age.

The lining of the roof of the mouth is essentially the same as that of the tongue. The glands are found only in the posterior region. The slight differences in the papillæ here found may easily be due to the difference in age.

The œsophagus shows the usual layers for that region. Its epithelium is partly ciliated in the anterior part. The muscularis mucosa is very scant in the anterior region. The only difference between the two stages is that in the feeding the muscularis mucosa in the anterior region is much more strongly developed than in the hibernating stage; and in the former the nuclei of the epithelium are not arranged in two zones as in the latter.

The stomach has the usual layers, and has essentially the same structure in the three regions studied, except that the wall in the fundic region is much the thickest, due mainly to the great thickness of the middle muscle layer. Only one kind of cell is found in the gastric glands. No difference is to be noted between the hibernating and feeding conditions.

The chief peculiarity of the small intestine is the apparent entire absence of the submucosa. Goblet cells are also wanting. The greater diameter of the anterior region is due both to the greater diameter of the lumen and to the greater thickness of the walls. The middle and posterior regions have about the same diameter, though the mucosa becomes thinner and less complicated caudad. There is practically no difference between the hibernating and feeding stages.

The anterior and posterior regions of the large intestine have essentially the same structure. No difference can be seen between the hibernating and feeding conditions.

CHAPTER VI
THE UROGENITAL ORGANS

Figure [54] represents the urogenital apparatus of a thirty-inch female specimen of Alligator mississippiensis. [Figure 55] shows the corresponding organs of a male A. lucius; reproduced from Bronn.

The urogenital organs in the young animal are so similar in the two sexes that one might easily be mistaken for the other; of course in sexually mature animals, especially during the breeding season, this is not the case.

The kidneys, [Fig. 54], k, [Fig. 55], a, are flattened, lobulated organs lying against the dorsal body wall. The large anterior lobe of each kidney is pointed at its anterior end and lies at some little distance from its fellow; it is partially divided into secondary lobes and is traversed on its ventral surface by branching blood-vessels. Its antero-medial border is sometimes partially concealed, in a ventral view, by the elongated gonad of that side. Caudad to the main lobe of the kidney is a smaller, usually distinct, lobe in contact mesially with its fellow of the opposite side.

A fairly wide ureter, [Fig. 54], u, [Fig. 55], d, extends from the posterior end of each kidney to open ([Fig. 54], u¹, [Fig. 55], e) into the anterior region of the cloaca, as described in connection with the digestive system.

The ovary, [Fig. 54], o, as noted above and as seen in [Figs. 54] and [55], in the young animal is of practically the same shape as the testis. The ova at this stage are of microscopic size and are hence not visible to the naked eye. The ovary, even at this stage, is more or less distinctly marked off into lobules by a series of small grooves.

Fig. 54. Female Urogenital System.

f, oviduct; f¹, opening of oviduct; k, kidney; m, mesentery; o, ovary; u, ureter; u¹, opening of ureter.

The oviduct, [Fig. 54], f, which at this stage is, of course, of small diameter, extends across the ventral surface of its corresponding kidney and opens, f¹, into the posterior part of the cloaca as has already been described. Its peritoneal opening is some distance cephalad to the head of the ovary. Its course from this opening is straight until about the anterior end of the ovary; it then becomes somewhat convoluted for a short distance, but gradually straightens out, to pass to its posterior end as a nearly straight duct. The anterior straight portion of the oviduct is connected with the head of the ovary by a narrow band of mesentery.

Each testis, [Fig. 55], b, like the ovary, lies along the ventro-mesial border of its corresponding kidney and is connected with the posterior region of the cloaca by a slender vas deferens, [Fig. 55], c, f.

According to Rathke (in C. acutus) a small, slender epididymis lies along the outer side of the posterior half of each testis.

The Copulatory Organs.

The penis, [Fig. 56], usually lies completely hidden in the cloaca; with its glans projecting backwards it is strongly arched; along the convex side of the arch, which is directed towards the upper wall of the cloaca, runs a groove, which serves as a penial urethra to conduct the semen.

According to Rathke there may be recognized in connection with the penis two fibrous strands (resembling the corpora cavernosa of mammals), a corpus cavernosus urethrea, and a covering derived from the mucous membrane of the cloaca. The two fibrous strands arise from the pubis as two thick plates that soon completely fuse together by their adjacent sides to form the shaft, c, of the penis. These fused strands taper gradually towards the glans, in which they end in a point. From their mode of fusion there is left between them, on the side towards the upper wall of the cloaca, a fairly deep furrow that extends to the tip. According to Rathke these shafts are not of cavernous tissue, but the tube is lined by a layer of this tissue.

Fig. 55. Male Urogenital Apparatus of Alligator Lucius. (After Bronn.)

a, kidney; b, testis; c, vas deferens; d, ureter; e, opening of the ureter into the cloaca; f, opening of the vas deferens into the cloaca; g, upper region of the cloaca; h, hinder region of the cloaca; i, rectum.

Fig. 56. Male Organ of Alligator Lucius, XI. (From Bronn, after Rathke.)

a, the right crus penis; b, the mucous membrane of the cloaca that covers the organ; c, shaft of the penis; d, base of glans; e, point of glans; f, part of the ring muscle of the cloaca.

The glans, e, consists of two parts between which, where they leave the shaft of the penis, is a funnel-shaped hole, wider towards the free end of the penis and divided into similar lateral halves by a fold of skin. The glans is much shorter than the shaft of the penis. The covering of the penis is much thinner than the mucous membrane of the cloaca and is thinnest along the groove; it extends from the shaft over the glans without forming a foreskin.

The base of the penis is attached to the pubis near its symphysis. With this base the most anterior part of the strong ring-muscle of the cloaca is closely attached by a fairly large mass of fibrous tissue. Rathke fails to find any muscles that are concerned alone with the copulatory organs.

In the copulation of the crocodile, according to Rathke, the penis is erected, though how this is caused is difficult to say since the corpora cavernosa consist only of fibrous tissue and the cavernous tissue lining the groove is very thin. The penis can, therefore, project only a short distance from the cloaca. The cavernous tissue is capable of causing only a slight elongation of the shaft, but the glans is considerably elongated by the strong influx of blood into that structure. According to Voeltzkow ([78]) the penis in the Madagascar crocodile is 20 cm. in length.

The clitoris of the sexually mature female crocodile is very much smaller than the penis of a male of the same size, but, according to Rathke, it varies greatly in size in different species. It is built on exactly the same plan as the penis.

According to Bronn the clitoris as well as the penis projects from the cloaca, out through the anus, in the embryo of the crocodile; this was not observed by the present writer in the embryo of the Florida alligator, A. mississippiensis.

CHAPTER VII
THE RESPIRATORY ORGANS

The Larynx and Trachea. In the Crocodilia the framework of the larynx consists of three cartilages, of which two represent the arytenoids of the Mammalia; the third represents the thyroid and cricoid of mammals. The last is considerably larger than the first and is a broad closed ring, differing in form in the different species. In spite of the fact that some of them have a voice, the vocal cords, according to Bronn, are wanting in the Crocodilia. According to Henle the vocal apparatus is produced by the projection into the laryngeal cavity of the inner border of the small arytenoid cartilages and by the infolding, under these cartilages, of the mucous membrane of the larynx; this forms the thick but fairly free folds that, when the glottis is narrowed, are well adapted to produce the harsh tone of the animal.

The epiglottis is absent in the Crocodilia.

In many Crocodilia (C. vulgaris, for example) the trachea, [Fig. 57], tr, forms a loop which begins in some species before hatching, in other species not until long after hatching. In the genus Alligator the trachea is straight.

More universal than this looped structure there is found another peculiar structure in the crocodilian trachea. It is a short vertical partition in the stem just before its division into the two branches. This partition is partly membranous and possesses one or more stiffening cartilaginous strands which are outgrowths of so many cartilaginous rings of the trachea. The number of the stiffening fibers varies in the different species.

Fig. 57. Respiratory Organs.

b, bronchus; e, œsophagus; g, glottis; l, lung; t, tongue; tr, trachea.

The number of the tracheal rings varies not only in the different species but also in different individuals of the same species. There are between fifty and sixty in A. mississippiensis. According to Rathke the number of rings in the individual animal almost certainly does not increase with age. The number of rings is smallest in the gavials and greatest in the crocodiles (genus Crocodilus). The number of rings in the two divisions of the trachea does not increase with age except, perhaps, in C. acutus and biporcatus. The lateral bend that the tracheal stem of so many Crocodilia exhibits is not due to the greater number of rings because in some species (gavials) where the bend is present the number of rings is smaller than in the Crocodilia where the bend is absent.

According to Rathke and others most of the tracheal rings are closed, but a varying, though at most small, number are open on the dorsal side. These openings become wider as the larynx is approached. The transverse muscle fibers which are found in the most anterior and largest of these breaks in the tracheal rings were found, says Rathke, in embryos after the middle period of incubation.

The cartilaginous rings of the bronchi, b, are also apparently open for a time after their formation, but soon close. Not infrequently in embryos and in young animals are found rings that are split like a fork, with one or both branches fused with neighboring branches.

The Lungs.

The lungs, [Fig. 57], l, are more highly developed among the Crocodilia than among any other Saurian or Hydrosaurian group. The histological groundwork of the whole lung tissue is a connective tissue of fine elastic fibers. In the lungs, on the canal that appears as the elongation of the bronchus, cartilage appears, according to Rathke, as bands lying one behind the other; some of these bands form complete, others partial rings; some of the latter are forked. The hindermost appear to be the broadest and most irregular. Their number is different in different species and varies in different individuals of the same species. They range in number, according to Rathke, from nine, in A. lucius, to twenty-five, in C. acutus.[7]

[7] Miller ([45c]) says: “In the crocodile and alligator the bronchus enters the lung near its center, and passes somewhat obliquely into the lung until it reaches the junction of the lower middle third; here it breaks up into eight to fifteen tubular passages. These tubular passages are studded with a great many air-sacs. In these animals the lung for the first time gives the structure as it is found in mammals. There are many air-sacs which communicate with a common cavity, or atrium, all of which in turn communicate with a single terminal bronchus. A single lobule of the mammalian lung is simply enlarged to form the lung of the crocodile; the lung of the former is only a conglomerate of that of the latter.”

The arterial branch, carrying venous blood to the lungs, develops a capillary network close to the alveolar walls, which leads away over the low alveolar septa, while over the tops of the higher septa and on the inner surface of the tube-like bronchial processes it forms a wide-meshed network of capillaries that are apparently chiefly nutrient.

All the respiratory capillaries are attached by only one side to the alveolar wall; the free side that projects into the air space of the alveolus is covered by a continuous pavement epithelium.

While the respiratory surfaces are covered with an alveolar epithelium of large polygonal cells, the free borders of all high septa and ridges, as well as the inner surfaces of the bronchial processes, are covered with ciliated cylindrical cells.

CHAPTER VIII
THE VASCULAR SYSTEM

The account given by Bronn in his Thierreich is apparently the only published description of the circulatory organs in the Crocodilia. This account, even when translated, is not very satisfactory, especially because it contains no diagrams of the circulation. It was, therefore, deemed worth while to work out the circulation in the Florida alligator in order that we might have not only a written description, but also a series of more or less accurate diagrams of the veins and arteries.

A number of departures from the description of Bronn were found, some of which are noted below.

Most of the work was done upon animals of about thirty inches length, which were obtained alive from the Arkansas Alligator Farm at Hot Springs, Ark.

The arteries were injected with a colored starch mass by inserting a two-way cannula into the dorsal aorta. With the blood thus forced into them from the arteries, the veins could, in most cases, be traced without difficulty.

In the diagrams the outlines of the more important organs are accurately shown by dotted lines, and the relative diameters of the blood-vessels are shown as accurately as possible by the solid black lines.