ix.j THE NERVOUS SYSTEM. 379 



cerebellum more than in man, as in the Saimiri, or they 

 may fail to cover that organ even in an Anthropoid Ape, i.e. 

 in the Siamang Gibbon. Generally it is widely uncovered, 

 and even in Mammals (e.g. in the Insectivorous Bats) the 

 cerebrum may leave the pineal gland and corpora quadri- 

 gemina uncovered also, as generally is the case in forms 

 below Mammals. 



The presence of lateral ventricles is characteristic of all 

 Vertebrates above Fishes, though the possession of tri-radiate 

 lateral ventricles is a character almost peculiar to Man and 

 Apes as only in the Seals besides is there a small posterior 

 cornu. Thus the posterior cornu appears, late in zoological 

 order as in chronological order, in man himself. Next to the 

 occipital lobe, the temporal one dwindles as we proceed 

 from man, the Sylvian fissure gradually opening outwards 

 till (as e.g. in the Rabbit) there is but a faint indication 

 of it. The cerebral 'hemispheres may be solid, as in Tele- 

 ostean Fishes, e.g. the Perch. 



That transverse commissure, the corpus callosum, may be 

 wanting, as in all below Mammals, and where it is absent 

 no fifth ventricle is, of course, enclosed. Pari passu with the 

 diminution of the corpus callosum we find generally in man's 

 class an increase in the relative size of the anterior commis- 

 sure, but below man's class the latter remains small in spite 

 of the absence of the former. Sometimes, however (as in 

 Cetacea), with only a fairly developed corpus callosum, the 

 anterior commissure may be almost obsolete. 



.3 2 



G' 



FIG. 331. RIGHT SIDE VIEW OF BRAIN OF THE COMMON LIZARD 



{Lacerta agilis). 



i, olfactory lobe ; 2, cerebral hemispheres ; 3, pineal gland ; 4, one of the optic 

 lobes ; 5, cerebellum ; 6, pituitary body ; 7, spinal marrow. 



12. The OLFACTORY LOBES of man are mere rudimentary 

 structures compared with their condition in many animals, as 

 e.g. in the Rabbit, and very often the ventricular cavity which 

 is but transitorily present in man, is permanent. They may 

 be sessile prominences, close to the hemispheres, as in the 

 Eels, or placed at the end of large diverging peduncles, as 

 in the Rays (Fig. 336, i). They may considerably exceed the 

 cerebral hemispheres in size, as in the Lamprey. 



