CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
735 
M ac i ‘oscopic A not only. 
The species which have been investigated for the purposes of this essay are Raja 
batis and clavata, Rhina squatina, Scyllium catulus and canicula, and Acanthias 
vulgaris. 
As the object of this paper is principally the microscopic anatomy, and only in a 
subordinate way is it devoted to the macroscopic appearance, a short description only 
of the external aspect of the brain will suffice, especially as good figures of many 
species are given in the works of Rohon and Miklucho-Maclay. 
The Rays are distinguished by the greater solidity and size of the central 
nervous system, particularly of the cerebellum. The ventricles are nearly obliterated, 
while in Acanthias, on the other hand, the ventricles are of grea.t size and have com¬ 
paratively thin walls, so that the brain has a deceptive appearance of great size. In 
the Rays the olfactory lobe is attached to the anterior external angle of the cerebrum 
by a peduncle, which becomes longer as the size, and presumably the age, of the 
animal increases. In Scyllium and Acanthias the corresponding peduncle is short, 
and the olfactory lobe is nearly sessile and projects but a short distance in front of the 
anterior end of the cerebrum ; in the former of these two species the olfactory lobe is 
of large size, almost equalling the cerebrum itself, while in Acanthias and Rhina it is 
comparatively small, and in the latter it is connected to the hemispheres by a some¬ 
what long but thin peduncle. 
The cerebrum in Acanthias and Rhina presents both dorsally and ventrally a longi¬ 
tudinal furrow, which is well developed anteriorly ; in Scyllium there is a dorsal 
furrow, but none on the ventral surface. In the Rays the surface is smooth, but in 
Scyllium the upper surface presents a pair of tuberosities. 
The cerebrum is attached to the anterior end of the optic lobes by a gutter-shaped 
neck of greater or less length, the thalamencephalon, of which the nervous tissue 
forms the floor and sides only ; the dorsal portion is covered by the pia mater, and 
gives access to processes from the same which form a veritable plexus choroides. The 
posterior edge of the thalamencephalon is marked by the attachment of the pineal 
gland (epiphysis) which in Scyllium is carried at the end of a long stalk, and when in 
position is placed in contact with the interior surface of the skull above the anterior 
end of the cerebrum. In Rhina the pia mater forms a vascular sac which covers over 
and projects above the level of the thalamencephalon. 
The mesencephalon, optic lobes, or corpora bigemina are two rounded prominences 
placed between the thalamencephalon in front and the cerebellum behind; they are 
separated from each other by a median longitudinal furrow. In Scyllium they are 
completely covered by the anterior lobe of the cerebellum; in the Rays this lobe 
projects only partially, and in Acanthias and Rhina scarcely at all over these segments 
of the brain. 
On the ventral or haemal aspect the hypoaria or lobi infundibuli are seen. They 
