(a) In some of the Muridæ, as Arvicola ([Fig. 394]), the mucous membrane of the large globular cæcal pouch is smooth, but the proximal segment of the colon, immediately beyond the ileo-colic junction, develops the spiral fold ([Fig. 465], IV, 2).
(b) In other forms, as in the hares ([Fig. 465], IV, 3), the greater part of the cæcum carries a typical spiral fold, continued up to the root of the terminal appendage ([Fig. 388]), in which segment the mucous membrane is devoid of folds, but studded thickly with lymphoid follicles. Beyond the cæcum proper the spiral fold is continued in the opposite direction into the proximal segment of the colon, which is large and capacious and evidently shares both the physiological and morphological characters of the cæcum proper, forming so to speak an accessory cæcal chamber. Beyond what we thus might term the cæcal division of the colon the large intestine becomes reduced in caliber, and the previously continuous spiral fold becomes broken up into separate semilunar haustral plicæ, corresponding to the superficial constrictions between the colic cells. In structure this distal segment of the rabbit colon closely resembles the human large intestine ([Fig. 474]).
One of the most marked examples of this secondary modification of the colon is presented by the intestinal canal of another Rodent, Lagomys pusillus ([Fig. 391]).
The cæcum of this animal is long, curved, provided with a well-developed spiral fold. The terminal segment of the pouch is reduced to an appendix, with smooth mucosa containing adenoid tissue, as in the rabbit. A second adenoid appendix, representing the globular saccus lymphaticus of the rabbit, is derived from the cæcum at the ileo-colic junction. The first segment of the colon beyond the ileo-colic junction is dilated and sacculated, the cæcal mucous fold being prolonged into it. This is succeeded by a narrow smooth-walled second segment. The third division of the colon is again dilated and sacculated, followed by a short fourth smooth-walled section. A fifth stretch is again provided with colic cells, beyond which the terminal segment continues of uniform caliber and with smooth walls to the vent. The colon therefore presents three distinct sacculated portions whose structural modifications suggest that they function in the same sense as the cæcal pouch proper. In man and in other Primates the crescentic colic plicæ are disposed in a more or less evident spiral manner around the axis of the intestine, and it is not difficult to recognize in them the modified remnants of the typical spiral valve of lower forms. On the other hand, in conformity with the general reduction of the cæcal apparatus, the mucous membrane of the large intestine in Carnivora is smooth and devoid of any trace of the spiral fold ([Fig. 475]).
2. The second structural modification of the large intestine, associated in functional significance with the cæcal apparatus, depends upon the increase in the length of the proximal segment of the colon beyond the ileo-colic junction and the twisting or coiling of this segment in a more or less complicated definite manner, usually in the form of a spiral, the individual turns of the coil being held in place by the peritoneal connections. The proximal colon thus modified is admirably adapted to retard the movement of contents not yet completely digested and to increase the absorbing surface of the intestine, and hence is functionally allied to the cæcal apparatus.
This colic modification is found in its highest degree of development in the ruminant Ungulates, whose cæcal pouch proper is also enormously developed. In these animals the colon immediately beyond the ileo-cæcal junction is arranged in the form of a double spiral, the afferent (cæcal) and efferent (colic) tubes alternating, and continuous with each other in the center of the coil ([Fig. 465], IV, 5). Examples of this type of spiral colon are shown in [Fig. 373] (Bos indicus), [Fig. 374] (Cervus sika), [Fig. 375] (Ovis aries), [Fig. 376] (Oryx leucoryx). Ontogenetically the complicated spiral colon of the ruminants starts as a simple loop of the proximal colon, which, with the further rapid growth of this segment of the intestine, is bent to produce the turns of the coil as shown in the schematic Figs. 480-482. Phylogenetically the same gradual development can be traced in the vertebrate series. Perhaps the earliest tendency to structurally modify the intestine in the direction named is found in the manner in which the intestinal coils are bound together by the subperitoneal arachnoid in many Ophidians ([Fig. 331]). Further in the Manidæ among the Edentates there is no cæcal pouch, but the intestine at the ileo-colic junction is twisted into a figure 8 and held in this position by the peritoneal connections ([Figs. 362] and [465], IV, 4). In certain Marsupials with well-developed cæcal pouches, such as Phascolarctos and the Vulpine Phalangers ([Figs. 351] and [352]), the colon immediately beyond the ileo-colic entrance is sacculated and bent in the form of a short loop. In the tapir ([Fig. 377]), the proximal segment of the colon forms a simple loop, whose afferent and efferent limbs are closely bound together. The arrangement of the large intestine in this animal illustrates the early embryonal stage in the development of the complete ruminant spiral coil (cf. [Fig. 480]).
| Figs. 480-482.—Schematic representation of three stages in the development of the ungulate spiral colon. |
The condition encountered in some Rodents presents a more advanced stage. Thus the large intestine in the agouti (Dasyprocta agouti), shows the development of the spiral coil advanced as far as the second turn of the original loop ([Figs. 389] and [390]). It is readily seen that continued growth of this segment of the intestine leads to the formation of the complete colic spiral as found in the typical Ungulates.
The same arrangement of the large intestine obtains in certain Lemurs among the Primates. Thus the proximal colon of the Slow Lemur (Nycticebus tardigradus) is seen in [Figs. 421] and [422] to present a typical spiral coil, and similar conditions are encountered in other members of the suborder.
V. Cæcal Apparatus and Colon in Hyrax.