-The Frog._

1. _General Anatomy._

Section 1. We will now study the adult anatomy of the frog, and throughout we shall make constant comparisons with that of the rabbit. In the rabbit we have a distinctly land-loving, burrowing animal; it eats purely vegetable food, and drinks but little. In the frog we have a mainly insectivorous type, living much in the water. This involves the moister skin, the shorter alimentary canal, and the abbreviated neck (Rabbit, [Section 2]) of the frog; the tail is absent-- in a fish it would do the work the frog accomplishes with his hind legs-- and the apertures which are posterior in the rabbit, run together into one dorsal opening, the cloaca. There is, of course (Rabbit, [Section 4]), no hair the skin is smooth, and an external ear is also absent. The remarkable looseness of the frog's skin is due to great lymph spaces between it and the body wall.

Section 2. If we now compare the general anatomy of the frog (vide [Sheet 11]) with that of the rabbit, we notice that the diaphragm is absent (Rabbit, [Section 4]), and the body cavity, or coelom, is, with the exception of the small bag of the pericardium round the heart, one continuous space. The forked tongue is attached in front of the lower jaw, and can be flicked out and back with great rapidity in the capture of the small insects upon which the frog lives. The posterior nares open into the front of the mouth-- there is no long nasal chamber, and no palate, and there is no long trachea between the epiglottis and the lungs. The oesophagus is less distinct, and passes gradually, so far as external appearances go, into the bag-like stomach, which is much less inflated and transverse than that of the rabbit. The duodenum is not a U-shaped loop, but makes one together with the stomach; the pancreas lies between it and the stomach, and is more compact than the rabbit's. There is no separate pancreatic duct, but the bile duct runs through the pancreas, and receives a series of ducts from that gland as it does so. The ileum is shorter, there is no sacculus rotundus, and the large intestine has no caecum, none of the characteristic sacculations of the rabbit's colon, and does not loop back to the stomach before the rectum section commences. The anus opens not upon the exterior, but into a cloacal chamber. The urinary and genital ducts open separately into this cloaca, and dorsally and posteriorly to the anus. The so-called urinary bladder is ventral to the intestine, in a position answering to that of the rabbit, but it has no connection with the ureters, and it is two-horned.

Section 3. The spleen is a small, round body, not so intimately bound to the stomach as in the rabbit, but in essentially the same position.

Section 4. Much that we knew of the physiology of the frog is arrived at mainly by inferences from our mammalian knowledge. Its histology is essentially similar. Ciliated epithelium is commoner and occurs more abundantly than in the rabbit, in the roof of the mouth for instance, and its red blood corpuscles are much larger, oval, and nucleated.

Section 5. The lungs of the frog are bag-like; shelves and spongy partitions project into their cavities, but this structure is much simpler than that of the rabbit's lung, in which the branching bronchi, the imperfect cartilaginous rings supporting them, alveoli, arteries and veins, form together a quasi-solid mass.

Section 6. The mechanism of respiration is fundamentally different from that of the mammal. The method is as follows:-- The frog opens its anterior nares, and depresses the floor of the mouth, which therefore fills with air. The anterior nares are then closed, and the floor mouth rises and forces the air into the lungs-- the frog, therefore, swallows its air rather than inhales it. The respiratory instrument of the rabbit is a suction pump, while that of the frog is a "buccal force pump."

Section 7. The heart is not quadrilocular (i.e., of four chambers), but trilocular (of three), and two structures, not seen in Lepus, the truncus arteriosus and the sinus venosus, into the latter of which the venous blood runs before entering the right auricle, are to be noted. The single ventricle is blocked with bars of tissue that render its interior, not an open cavity, but a spongy mass. Figure 2, [Sheet 11], shows the heart opened; l.au. and r.au. are the left and right auricles respectively; the truncus arteriosus is seen to be imperfectly divided by a great longitudino-spiral valve (l.s.v.); p.c. is the pulmo-cutaneous artery -going to the lungs- [supplying skin and lungs]; d.ao., the dorsal aorta [furnishing the supply of the body and limbs]; and c.a. the carotid artery going to the head; all of which vessels (compare [Figure 1]) are paired.

Section 8. It might be inferred from this that pure and impure blood mix in the ventricle, and that a blood of uniform quality flows to lungs, head, and extremities; but this is not so. The spongy nature of the ventricle sufficiently retards this mixing. It will be noted that the opening of pulmonary arteries lies nearest to the heart, next come the aortic and carotid arches, which have a common opening at A. Furthermore, at c.g.l. [the carotid artery, repeatedly divides to form a close meshwork of arterioles, the carotid gland, forming a sponge-like plug in this vessel.] is a spongy mass of matter, the carotid gland inserted upon the carotid. Hence the pulmonary arteries yawn nearest for the blood, and, being short, wide vessels, present the least resistance to the first rush of blood-- mainly venous blood for the right auricle. As they fill up, the back resistance in them becomes equal, and then greater, than the resistance at A, and the rush of blood, now of a mixed quality passes through that aperture. It selects the dorsal aorta, because the carotid arch, plugged by the carotid gland, offers the greater resistance. Presently, however, the back resistance of the filled dorsal aorta rises above this, and the last flow of blood, from the ventricular systole-- almost purely oxygenated blood for the left auricle-- goes on towards the head.

Section 9. At the carotid gland the carotid artery splits into -an- [a] -external carotid- [lingual] (e.c.), and a deeper internal carotid. The dorsal aorta passes round on each side of the oesophagus, as indicated by the dotted lines in Figure 2, [Sheet 11], and meets its fellow dorsal to the liver. Each arch gives off subclavian arteries to the limbs, and the left, immediately before meeting the right, gives off the coeliaco-mesenteric artery [to the alimentary canal]. This origin of the coeliaco-mesenteric artery a little to the left, is the only asymmetry (want of balance) in the arterial system of the frog, as contrasted with the very extensive asymmetry of the great vessels near the heart of the rabbit. [Posteriorly the dorsal aorta forks into two common iliac arteries (right and left) supplying the hind limbs.]

Section 10. [Figure 3] gives a side view of the frog, to display the circulation.

{Lines from Second Edition only.}
[The venous return to the heart, as in the rabbit, is by paired venae cavae anteriores and by a single vena cava inferior. The factors of the anterior cava on either side are an external jugular (ex.j.) an innominate vein (in.v.) and subclavian (scl.v.). The last receives not only the brachial vein (b.v.) from the fore limb, but also a large vein bringing blood for the skin, the cutaneous (p.v.). The innominate vein has also two chief factors, the internal jugular (l.i.j.v.) and the subscapular (s.s.v.). The blood returns from each hind limb by a sciatic (l.sc.) or femoral (f.m.) vein, and either passes to a renal portal vein (l.r.p.), which breaks into capillaries in the kidney, or by a paired pelvic vein (l.p.v. in [Figures 1 and 3]) which meets its fellow in the middle line to form the anterior abdominal vein (a.ab.v.) going forward and uniting with the (median) portal vein (p.v.) to enter the liver.]

-The vessels are named in the references to the figure, which should be carefully copied and mastered. Here we need only- [Comparing with the rabbit, we would especially] call attention to the fact that the vena cava inferior extends posteriorly only to the kidney, and that there is a renal portal system. The blood from the hind limbs either flows by the anterior abdominal vein to the portal vein and liver, or it passes by the renal portal vein to the kidney. There the vein breaks up, and we find in the frog's kidney, just as we find in the frog's and rabbit's liver, a triple system of (a) nutritive arterial, (b) afferent* venous and (c) efferent** venous vessels.

* a, ad = to;
** e, ex = out of.

{This Section missing from Second Edition.}
-Section 11. It is not very improbable that the kidney of the frog shares, or performs, some of the functions of the rabbit's liver, or parallel duties, in addition to the simply excretory function. Since specialization of cells must be mainly the relatively excessive exaggeration of some one of the general properties of the undifferentiated cell, it is not a difficult thing to imagine a gradual transition, as we move from one organism to another, of the functions of glands and other cellular organs. It is probable that the mammalian kidney is, physiologically, a much less important (though still quite essential) organ than the structures which correspond to it in position and development in the lower vertebrate types.-

Section 12. The lymphatic system is extensively developed in the frog, but, in the place of a complete system of distinctly organized vessels, there are great lymph sinuses (compare [Section 1]). In Figure 5, [Sheet 12], the position of two lymph hearts (l.h., l.h.) which pump lymph into the adjacent veins, is shown.

Section 13. The skull of the frog will repay a full treatment, and will be dealt with by itself later. The vertebral column ([Sheet 12]) consists of nine vertebrae, the centra of which have faces, not flat, but hollow in front (pro-coelous), and evidently without epiphyses (compare the Rabbit). The anterior is sometimes called the atlas, but it is evidently not the homologue of the atlas of the rabbit, since the first spinal nerve has a corresponding distribution to the twelfth cranial of the mammal, and since, therefore, it is probable that the mammalian skull = the frog's skull + one (or more) vertebrae incorporated with it. Posteriorly the vertebral column terminates in the urostyle, a calcified unsegmented rod. The vertebrae have transverse processes, but no ribs.

Section 14. The fore-limb (Figure 6, [Sheet 12]) consists of an upper segment of one bone, the humerus, as in the rabbit; a middle section, the radius and ulna, fused here into one bone, and not, as in the mammalian type, separable; of a carpus, and of five digits, of which the fourth is the longest. The shoulder girdle is more important and complete than that of the higher type. There is a scapula (sc.) with an unossified cartilaginous supra-scapula (s.sc.); the anterior border of the scapula answers to the acromion. On the ventral side a cartilaginous rod, embraced by the clavicle (cl.) (a membrane bone in this type), runs to the sternum, and answers to the clavicle of the rabbit. In the place of the rabbit's coracoid process, is a coracoid bone (co.), which reaches from the glenoid cavity to the sternum; it is hidden on the right side of [Figure 6], which is a dorsal view of the shoulder girdle. There is a pre-omosternum (o.st.) and a post-omosternum, sometimes termed a xiphisternum (x.).

Section 15. [Figure 7] shows the pelvic girdle and limb of the frog. There is a femur (f.); tibia and fibula (t. and f.) are completely fused; the proximal bones of the tarsus, the astragalus (as.), and calcaneum (cal.) are elongated, there are five long digits, and in the calcar (c.) an indication of a sixth. With considerable modifications of form, the three leading constituents of the rabbit's pelvic girdle occur in relatively identical positions. The greatly elongated ilium (il.) articulates with the single (compare Rabbit) sacral vertebra (s.v. in [Figure 5]). The ischium (is.) is relatively smaller than in the rabbit, and the pubis (pu.) is a ventral wedge of unossified cartilage. The shape of the pelvic girdle of the frog is a wide departure from that found among related forms. In connection with the leaping habit, the ilia are greatly elongated, and the pubes and ischia much reduced. Generally throughout the air-frequenting vertebrata, we find the same arrangement of these three bones, usually in the form of an inverted. Y-- the ilium above, the ischium and pubis below, and the acetabulum at the junction of the three.

Section 16. The uro-genital organs of the frog, and especially those of the male, correspond with embryonic stages of the rabbit. In this sex the testes (T., [Sheet 13]) lie in the body cavity, and are white bodies usually dappled with black pigment. Vasa efferentia (v.e.) run to the internal border of the anterior part of the kidney, which answers, therefore, to the rabbit's epididymis. The hinder part of the kidney is the predominant renal organ. There is a common uro-genital duct, into which a seminal vesicle, which is especially large in early spring, opens. This is the permanent condition of the frog. In the rabbit, for urogenital duct, we have ureter and vas deferens; the testes and that anterior part of the primitive kidney, the epididymis, shift back into the scrotal sacs, and the ureters shift round the rectum and establish a direct connection with the bladder, carrying the genital ducts looped over them. The oviducts of the female do not fuse distally to form a median vagina as they do in the rabbit. In front of the genital organ in both sexes is a corpus adiposum (c.ad.), which acts as a fat store, and is peculiar to the frogs and toads. The distal end of the oviduct of the female is in the breeding season (early March) enormously distended with ova, and the ovaries become then the mere vestiges of their former selves. The distal end of the oviduct is, therefore, not unfrequently styled the uterus. There is no penis in the male, fertilisation of the ova occurring as they are squeezed out of the female by the embracing fore limbs of the male. The male has a pad, black in winter, shown in [Figure 1], which is closely pressed against the ventral surface of the female in copulation, and which serves as a ready means of distinguishing the sex.

Section 17. The spinal cord has a general similarity to that of the rabbit; the ratio of its size to that of the brain is larger, and the nerves number ten pairs altogether. The first of these (sp. 1, in Figure 2, [Sheet -12-] ) {First Edition error.} [13] corresponds in distribution with the rabbit's hypoglossal nerve, a point we shall refer to again when we speak of the skull. The second and third constitute the brachial plexus. The last three form the sciatic plexus going to the hind limb.

Section 18. The same essential parts are to be found in the brain of both frog and rabbit, but in the former the adult is not so widely modified from the primitive condition as in the latter. The fore-brain consists of a thalamencephalon (th.c. and 1), which is exposed in the dorsal view of the brain, and which has no middle commissure. The cerebral hemispheres (c.h.) are not convoluted, do not extend back to cover parts behind them, as they do in the rabbit, and are not connected above the roof of the thalamencephalon by a corpus callosum. Moreover, the parts usually regarded, as the olfactory lobes (rh.) fuse in the middle line. The mid-brain gives rise to the third nerve, and has the optic lobes on its dorsal side, but these are hollow, and they are not subdivided by a transverse groove into corpora quadrigemina, as in the rabbit. In the hind-brain the cerebellum is a mere band of tissue without lateral lobes or flocculi, and the medulla gives origin only to nerves four to ten; there is no eleventh nerve, and the hypoglossal is the first spinal-- from which it has been assumed that the rabbit's medulla equals that of the frog, plus a portion of the spinal cord incorporated with it. The hypoglossal is very distinctly seen on opening the skin beneath the hyoid plate.

Section 19. The first, second, third, and fourth cranial nerves of the frog correspond with those of the rabbit in origin and distribution. So do five, six and eight. The seventh nerve forks over the ear-drum-- the larger branch emerging behind it and running superficially, as shown in [Figure 4]. There is also a deeper palatine branch of VII. (P.) running under V2 and V3 below the orbit, and to be seen together with V1 and V2 after removal of the eyeball. The ninth nerve similarly forks over the first branchial slit of the tadpole, and evidence of the fork remains in the frog. It is seen curving round anterior to the hypoglossal nerve, and lying rather deeper in dissection. The vagus (tenth) nerve is distributed to heart, lungs, and viscera-- in the tadpole it also sends for forking branches over the second, third, and fourth branchial slits. It lies deeper than IX., and internal to the veins, and runs close beside the cutaneous artery. Most of these nerves are easily dissected and no student should rest satisfied until he has actually seen them.

Section 20. The sympathetic chain is closely connected with the aorta. It is, of course, paired, and is easily found in dissection by lifting the dorsal aorta and looking at its mesentery. In the presence of ganglia corresponding to the spinal nerves, and of rami communicantes, it resembles that of the rabbit.

Section 21. The whole of this chapter is simply a concise comparison, of frog and rabbit. In addition to reading it, the student should very carefully follow the annotations to the figures, and should copy and recopy these side by side with the corresponding diagrams of the other types.

2. _The Skull of the Frog (and the vertebrate skull generally)_

Section 22. We have already given a description of the mammalian skull, and we have stated where the origin of the several bones was in membrane, and where in cartilage; but a more complete comprehension of the mammalian skull becomes possible with the handling of a lower type. We propose now, first to give some short account of the development and structure of the skull of the frog, and then to show briefly how its development and adult arrangement demonstrate the mammalian skull to be a fundamentally similar structure, complicated and disguised by further development and re-adjustment.

Section 23. Figure 1,I. [Sheet 14], shows a dorsal view of a young tadpole cranium; the brain has been removed, and it is seen that it was supported simply upon two cartilaginous rods, the trabeculae cranii (tr.c.). Behind these trabeculae comes the notochord (n.c.), and around its anterior extremity is a paired tract of cartilage, the parachordals (p.c.). These structures, underlying the skull, are all that appear at first of the brain box. In front, and separate from the cranium, are the nasal organs (n.c.); the eyes lie laterally to the trabeculae, and laterally to the parachordals are two tracts of cartilage enclosing the internal ear, the otic capsules.

Section 24. [Figure 1, II.], is a more advanced, phase of the same structures. The trabeculae have met in front and sent forward a median (c.t.) and lateral parts (a.o.) to support the nasal organs. They have also flattened, out very considerably, and have sent up walls on either side of the brain to meet above it and form an incomplete roof (t.) over it. The parachordals have similarly grown up round, the hind-brain and formed a complete ring, the roof of which is indicated, by b. Further, the otic capsules are fusing with the brain-case. With certain differences of form these elements-- the trabeculae, the parachordals, and the otic capsules, are also the first formed structures of the mammalian cranium.

Section 25. In [Figures 1,I. and II.], there appears beneath the eye a bar of cartilage (p.p.), the palato-pterygoid cartilage, which is also to be seen from the side in [Figures 8,I. and III]. It will be learnt from these latter that this bar is joined in front to the cranium behind the nasal organ, and behind to the otic capsule. The cartilaginous bar from the palato-pterygoid to the otic capsule is called the quadrate, and at the point of junction, at the postero-ventral angle of the palato-pterygoid, articulates with the cartilaginous bar which is destined to form the substratum of the lower jaw-- Meckel's cartilage (M.c. in [Figure 8,I.]).

Section 26. [Figure 2] shows a dorsal view of these structures in a young frog. The parts corresponding to these in 1,II. will be easily made out, but now ossification has set in at various points of this cartilaginous cranium. In front of the otic capsule is the paired pro-otic bone (p.o.); behind it at the sides of the parachordal ring is the paired ex-occipital (e.o.); in front of the cranium box, and behind the nasal capsules, is a ring of bone, the (median, but originally paired) sphenethmoid (s.e.). -A paired ossification appears in the palato-pterygoid cartilage the pterygoid bone (pt.), while- A splint of bone, the quadrato-jugal, appears at the angle of articulation with the lower jaw. These are all the cartilage bones that appear in the cranium and upper jaw of the frog.

Section 27. But another series of bones, developed first chiefly in dermal connective tissue, and coming to plate over the cranium of cartilage, are not shown in Figure 2. They are, however, in [Figure 3]. These membrane bones are: along the dorsal middle line, the parieto-frontals (p.f.), originally two pairs of bones which fuse in development, and the nasals (na.). Round the edge of the jaw, and bearing the teeth, are pre-maxillae (p.m.), and maxillae (mx.), and overlying the quadrate cartilage and lateral to the otic capsules are the T-shaped squamosal bones (sq.). In the ventral view of the skull ([Figure 4]) we see a pair of vomers (vo.) bearing teeth, a pair of palatines (pal.), [and a pair of pterygoids (pt.)] (which [palatines and pterygoids, we may note,] unlike those of the rabbit, are -stated to be- membrane bones), and a great median dagger-shaped para-sphenoid (p.sp.). These two Figures, and [5], which shows the same bones in side view, should be carefully mastered before the student proceeds with this chapter. The cartilage bones are distinguished from membrane bones by cross-shading.

Section 28. Turning now to [Figure 8,I.], we have a side view of a tadpole's skull. On the ventral side of the head is a series of vertical cartilaginous bars, the visceral arches supporting the walls of the tadpole's gill slits. The first of these is called the hyoid arch (c.h.), and the four following this, the first (br.1), second, third, and fourth (br.4), branchial arches. Altogether there are four gill slits and between the hyoid arch and the jaw arch, as it is called (= Meckel's cartilage + the palato-pterygoid), is "an imperforate slit," which becomes the ear-drum.* The frog no longer breathes by gills, but by lungs, and the gills are lost, the gill slits closed, and the branchial arches consequently much reduced. [Figures 8, II., and 8, III.], show stages in this reduction. The hyoid arch becomes attached, to the otic capsule, and its median ventral plate, including also the vestiges of the first, second, and fourth branchial arches, is called the hyoid apparatus. In [Figure 5], the apparatus is seen from the side; c.h. is called the (right) anterior cornu** of the hyoid. The function of the hyoid apparatus in the frog is to furnish, a basis of attachment to the tongue muscles; it remains cartilaginous, with the exception of the relic of one branchial arch, which ossifies as the thyro-hyal ([Figure 7] th.h.). It will be noted that, as development proceeds, the angle of the jaw swings backward, and the hyoid apparatus, shifts relatively forward. These changes of position are indicated in [Figure 8, III.], by little arrow-heads.

* We may note here that, comparing the ear of the frog with that of the rabbit, there is no external ear. There is, moreover, no bulla supporting the middle ear, and the tympanic membrane stretches between the squamosal in front and the anterior cornu of the hyoid behind. A rod-like columella auris replaces the chain of ear ossicles, and may, or may not, answer to the stapes alone, or even possibly to the entire series. In the internal ear there is no cochlea, and the otic mass is largely cartilaginous instead of entirely bony.

** Plural cornua.

Section 29. Before proceeding to the comparison of the mammalian skull with this, we would strongly recommend the student thoroughly to master this portion of the work, and in no way can he do this more thoroughly and quickly than by taking a parboiled frog, picking off the skin, muscle, and connective tissue from its skull, and making out the various bones with the help of our diagrams.

Section 30. [Figure 9] represents, in the most diagrammatic way, the main changes in form of the essential constituents of the cranio-facial apparatus, as we pass from the amphibian to the mammalian skull. F. is the frog from the side and behind; b.c. is the brain-case, o.c. the otic capsule, e. the eye, n.c. the nasal capsule, p.p. the palato-pterygoid cartilage, mx. the maxillary membrane bones, sq. the squamosal, and mb. the mandible. The student should compare with [Figure 5], and convince himself that he appreciates the diagrammatic rendering of these parts. Now all the distinctive differences in form, from this of the dog's skull (D.), are reducible to two primary causes--

(1) The brain is enormously larger, and the brain-case is vastly
inflated, so that--

(a) the otic capsule becomes embedded in the brain-case wall;
(b) the palato-pterygoid rod lies completely underneath the brain-case instead of laterally to it;
(c) the squamosal tilts down and in, instead of down and out, and the lower jaw articulates with its outer surface instead of below its inner, and, moreover, with the enormous distention of the brain-case it comes about that the squamosal is incorporated with its wall.

(2) The maxilla anteriorly and the palatine posteriorly send down palatine plates that grow in to form the bony palate, cutting off a nasal passage (n.p.) from the mouth cavity (m.p.), and carrying the posterior nares from the front part of the mouth, as they are in the frog, to the pharynx. Hence the vomers of the dog lie, not in the ceiling of the mouth, but in the floor of this nasal passage.

Section 31. The quadrate cartilage of the frog is superseded by the squamosal as the suspensorium of the lower jaw. It is greatly reduced, therefore; but it is not entirely absent. In the young mammal, a quadrate cartilage can be traced, connected with the palato-pterygoid cartilage, and articulating with Meckel's cartilage. Its position is, of course, beneath the squamosal, and just outside the otic capsule. As development proceeds, the increase in size of the quadrate, does not keep pace with that of the skull structures. It loses its connection with the palato-pterygoid, and apparently ossifies as a small ossicle-- the incus of the middle ear. A small nodule of cartilage, cut off from the proximal end of Meckel's cartilage, becomes the malleus. The stapes would appear to be derived from the hyoid arch. Hence these small bones seem to be the relics of the discarded jaw suspensorium of the frog utilized in a new function. Considerable doubt, however, attaches to this interpretation-- doubt that, if anything, is gaining ground.

Section 32. The tympanic bulla of the dog is not indicated in Diagram 9, and it would appear to be a new structure (neomorph), not represented in the frog.

Section 33. Besides these great differences in form, there are important differences in the amount and distribution of centres of ossification of the skull of frog and mammal. There is no parasphenoid in the mammal*; and, instead, a complete series of ossifications, the median-, basi-, and pre-sphenoids, and the lateral ali- and orbito-sphenoids occur. The points can be rendered much more luminously in a diagram than in the text, and we would counsel the student to compare this very carefully with that of the Rabbit.

* Faint vestigial indications occur in the developing skulls of some insectivora.

Section 34.

-Cranium_

-Nasal_ (paired), -Vomer_ (paired)
-Fronto-Parietal_, Sphenethmoid Bone (median), Eye, Pro-otic Bone,
Otic Cartilage, Ex-occipital (paired)
-Para-sphenoid Bone_

-Upper Jaw_

-Pre-Maxilla_ (paired), -Palatine_ (paired), Pterygoid (paired),
-Squamosal_, Quadrate Cartilage {To 1.}
-Maxilla_
1. Quadrato-Jugal

<-Lower Jaw_

Mento-meckelian, -Dentary_, -Articulare- [-Angulo Splenial_]

Section 35. -Points especially- [Additional points] to be noticed are:

(1) The otic capsule (= periotic bone) of the dog ossifies from a number of centres, one of which is equivalent to the frog's prootic.
(2) The several constituents of the lower jaw are not to be distinguished in the adult mammal.
(3) The frog has no lachrymal bone.

Section 36. We are now in a position to notice, without any danger of misconception, what is called the segmental theory of the skull. Older anatomists, working from adult structure only, conceived the idea that the brain-case of the mammal represented three inflated vertebrae. The most anterior had the pre-sphenoid for its body, the orbito-sphenoids for its neural processes, and the arch was completed above by the frontals (frontal segment). Similarly, the basi-sphenoids, ali-sphenoids, and parietals formed a second arch (parietal segment), and the ex-, basi-, and supra-occipitals a third (occipital segment). If this were correct, in the frog, which is a more primitive rendering of the vertebrate plan, we should find the vertebral characters more distinct. But, as a matter of fact, as the student will perceive, frontal segment, parietal segment, and occipital segment, can no longer be traced; and the mode of origin from trabeculae and para-chordals show very clearly the falsity of this view. The vertebrate cranium is entirely different in nature from vertebrae. The origin of the parietals and frontals as paired bones in membrane reinforces this conclusion.

Section 37. But as certainly as we have no such metameric segmentation, as this older view implies, in the brain-case of the frog, so quite as certainly is metameric segmentation evident in its branchial arches. We have the four gill slits of the tadpole and their bars repeating one another; the hyoid bar in front of these is evidently of a similar nature; and that the ear drum is derived from an imperforate gill slit is enforced by the presence of an open slit (the spiracle) in the rays and dog-fish in an entirely equivalent position. Does the mouth answer to a further pair of gill slits, and is the jaw arch (palato-pterygoid + Meckel's cartilage) equivalent to the arches that come behind it? This question has been asked, and answered in the affirmative, by many morphologists, but not by any means by all. The cranial nerves have a curious similarity of arrangement with regard to the gill slits and the mouth; the fifth nerve forks over the mouth, the seventh forks over the ear drum, the ninth, in the tadpole and fish, forks over the first branchial slit, and the tenth is, as it were, a leash of nerves, each forking over one of the remaining gill slits. But this matter will be more intelligible when the student has worked over a fish type, and need not detain us any further now.

Section 38. See also [Section 13] again, in which is the suggestion that the occipital part of the skull is possibly a fusion of vertebrae, a new view with much in its favour, and obviously an entirely different one from the old "segmental" view of the entire skull, discussed in Section 36.

2. _Questions on the Frog_

[All these questions were actually set at London University Examinations.] {In Both Editions.}

  1. Give an account, with illustrative sketches, of the digestive organs of the common frog, specifying particularly the different forms of epithelium met with in the several regions thereof.
  2. Describe the heart of a frog, and compare it with that of a fish and of a mammal, mentioning in each case the great vessels which open into each cavity.
  3. Compare with one another the breathing organs and the mechanism of respiration in a frog and in a rabbit. Give figures showing the condition of the heart and great arteries in these animals, and indicate in each case the nature of the blood in the several cavities of the heart.
  4. Draw diagrams, with the parts named, illustrating the arrangement of the chief arteries of (a) the frog, (b) the rabbit. (c) Compare briefly the arrangements thus described. (d) In what important respects does the vascular mechanism of the frog differ from that of the fish, in correlation with the presence of lungs?
  5. In the frog provided, free the heart, both aortic arches, dorsal aorta as far as its terminal bifurcation, and both chains of sympathetic ganglia from surrounding structures; and remove them, in their natural connection, from the animal into a watch-glass.
  6. Describe the male and female reproductive organs of the common frog, and give some account of their development.
  7. Describe, with figures, the bones of the limbs and limb-girdles of a frog.
  8. Remove the brain from the frog provided, and place it in spirit. Make a lettered drawing of its ventral and dorsal surfaces.
  9. Point out the corresponding regions in the brain of a frog and a mammal, and state what are the relations of the three primary brain-vesicles to these regions.
  10. (a) Give an account, with diagrams, of the brain of the frog; (b) point out the most important differences between it and the brain of the rabbit. (c) Describe the superficial origin and the distribution of the third, (d) of the fifth, (e) of the seventh., (f) of the ninth, and (g) of the tenth cranial nerves of the frog.
  11. Describe, with figures, the brain of a frog, and compare it with that of a rabbit. What do you know concerning the functions of the several parts of the brain in the frog?
  12. Describe briefly the fundamental properties of the spinal cord in the frog. By what means would you determine whether a given nerve is motor or sensory?
  13. Prepare the skull of the frog provided. Remove from it and place in glycerine on a glass slip the fronto-parietal and parasphenoid bones. Label them. Mark on the skull with long needles and flag-labels the sphenethmoid and the pro-otic bones.
  14. Compare the skull of the rabbit and the frog; especially in regard to the attachment of the jaw apparatus to the cranium, and other points which distinctly characterize the higher as contrasted with the lower vertebrata.
  15. Describe the skeleton of the upper and lower jaw (a) in the frog, (b) in the rabbit. Point out exactly what parts correspond with one another in the two animals compared. (c) What bone in the rabbit is generally regarded as corresponding to the quadrate cartilage of the frog?