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 wi