236 COMPARATIVE ANATOMY 



wall of the stomodaeum, and its posterior end, being attached 

 to the floor of the skull, retains a trace of its origin. 



The corpora bigemina are prominent rounded swellings on 

 the dorsal surface of the mid-brain, and are partly overlapped 

 by the anterior end of the cerebellum. They contain cavities 

 which open below into the so-called Sylvian aqueduct, leading 

 from the third to the fourth ventricle. The cerebellum of the 

 dogfish is relatively of large size, and very unlike the corre- 

 sponding organ in the frog. Viewed from above, it is oval in 

 outline, and it partly overhangs the corpora bigemina in front 

 and the medulla oblongata behind. Its dorsal surface is 

 marked with a shallow, longitudinal furrow, dividing it into 

 right and left lobes, but it contains a single wide cavity which 

 communicates by a narrower passage with the fourth ventricle 

 below. At the sides of the cerebellum are the restiform 

 bodies, a pair of hollow outgrowths of the dorso-lateral walls 

 of the hind-brain, with thin and much-folded walls. They 

 communicate with one another below the hinder extension 

 of the cerebellum by a band of nervous tissue bridging over 

 the anterior part of the fourth ventricle. 



The medulla oblongata is narrow in front and behind, 

 where it passes into the mid-brain and the spinal cord re- 

 spectively, and widest in its middle part. Its floor and sides 

 are thick, but its roof is very thin, consisting of a transparent 

 sheet of tissue, the velum medullae posterius, covering in the 

 wide cavity of the fourth ventricle. The last-named narrows 

 posteriorly, and is continued into the canal of the spinal 

 cord. 



The dogfish has ten pairs of cranial nerves which in dis- 

 tribution and arrangement closely resemble those of the frog, 

 but as the dogfish breathes by gills, while the adult frog 

 breathes by lungs, we find that the more posterior pairs of 

 cranial nerves of the former animal bear relations to the 

 gill-arches which are worthy of the most careful study. 



The olfactory nerves arise as a number of fine branches 

 from the anterior ends of the olfactory lobes, and pass at 

 once into the nasal capsules, where they are distributed to 

 the epithelium lining the olfactory organ. 



The optic nerves arise from the lower surface of the 

 thalamencephalon. The nerve of each side crosses its fellow 

 in the optic chiasma, and passing to the opposite side of the 



