ENCEPHALON OF THE PERCH. 663 



(1834.) In the higher quadrupeds, and more especially in Man, the 

 proportionate size of the hemispheres of the brain is so enormous that 

 they overlap and conceal all the parts we have been describing ; but as 

 we descend to lower forms their relative dimensions become gradually 

 smaller and their structure less complicated, until in fishes, the least 

 intelligent of all the creatures belonging to this great division of the 

 animal kingdom, they are found in such a rudimentary condition that 

 they are frequently far inferior in size even to the olfactory or optic 

 ganglia (fig. 322, c). 



(1835.) The lobes representing the hemispheres in fishes (fig. 324, 6) 

 are quite smooth externally, and within are hollowed into a large ven- 

 tricle, in the floor of which is seen the upper surface of the optic 

 ganglia (fig. 324, B, d). They present none of that complication of 

 parts met with in the brains of higher orders : their inner surface is 

 lined with transverse fibres (7i), and a simple commissure passes across 

 the anterior part of the ventricle, bringing the two sides into communi- 

 cation with each other ; behind the commissure a passage leads to the 

 third ventricle, the infundibulum, and the pituitary gland. 



Fig. 324. 



ABC 



Brain of the Perch (after Cuvier) : a, cerebellum ; b, cerebrum ; c, olfactory ganglia ; 

 i, olfactory nerves; d, optic ganglia; g, supplementary lobe; h, transverse fibres in the 

 walls of the cerebral ventricle; n, commissure of the optic nerves; p, q, r, e, t, , the 

 third, fourth, fifth, sixth, seventh, and eighth pairs of cerebral nerves. 



(1836.) The cerebellum (fig. 324, a) is at once recognizable from its 

 position and singleness. In the Perch its form is that of a blunted cone, 

 with the summit directed slightly backward ; but the shape and relative 

 dimensions of this part of the brain are extremely variable. It consists, 

 in fishes, only of the central portion (processus vermiformis) , so that 

 there are neither lateral lobes nor pans Varolii : its surface is composed 

 of cineritious substance, and in its centre is a ramified medullary axis 

 containing a ventricle. 



(1837.) One very remarkable feature in the structure of the ence- 

 phalon of fishes is the existence of supplementary lobes (fig. 324, g), 



