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.