32 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



and left cerebral hemispheres (Fig. 15). The trench that 

 divides them is about two inches deep and occupied by 

 an extension of the membranes. The two hemispheres 

 are connected below by a band about half an inch thick 

 —the corpus callosum. The cerebrum constitutes about 

 four fifths of the whole brain. 



2. Cerebellum. — Lifting the cerebrum from behind, 

 the next most important part brought into view imme- 

 diately beneath the 

 hinder part of the 

 cerebrum is the cere- 

 bellum. This is also 

 a double organ like 

 the cerebrum, but the 

 two halves are not 

 so deeply divided by 

 the membranes. The 

 peculiar leaflike ar- 

 rangement of the 

 convolutions will be 

 observed. A side 

 view of the brain 

 (Fig. 16) shows the 



cerebellum beneath the under part of the cerebrum. 



3. Medulla and Pons. — Lying beneath the cere- 

 bellum, as if this latter had grown out of it, is an en- 

 larged continuation of the spinal cord within the skull. 

 This is called the medulla. Beneath this again, with 

 fibers running across the medulla and connecting the 

 two sides of the cerebellum, is the pons Varolii (bridge of 

 Varolius). This can only be seen, however, by turning 

 the brain over so as to see the under side (Fig. 17). 



4. Optic Lobes. — Lifting the cerebrum from behind 

 still higher and looking farther forward, the optic lobes 

 are brought into view in front of the cerebellum. In 



Fig. 16. — Side view of the brain : cr, cere- 

 brum ; cb, cerebellum ; m y medulla ; s, fis- 

 sure of Sylvius ; r, fissure of Rolando. 



