Chapter CX: 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[s] 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.