THE BRx^IN 79 



The Brain 



Superficial examination of the brain of the rabbit (Fig. 42) 

 shows that its larger part is formed by the paired cerebral hemi- 

 spheres. They are closely pressed together on the dorsal side but 

 separated posteriorly on the ventral side. The external layer, the 

 grey cortex, is important as the principal part of the physical 

 substratum of intelligence. In the rabbit, as in all animals which 

 have not large cerebral hemispheres, the cortex is smooth, con- 

 trasting with its condition in mammals with larger hemispheres, 

 where it is corrugated. The amount of this corrugation is roughly 

 related to the absolute size of the brain. Thus it is considerable 

 in the cat and dog but reaches a high degree of elaboration in man 

 and in the whale. At the anterior tips of the cerebral hemispheres 

 in the rabbit are the slightly expanded olfactory bulbs which can 

 be seen to be connected backwards with the posteroventral portion 

 of the hemisphere, known as the pyriform lobe. These parts of the 

 brain being directly related to the sense of smell, their size in 

 different mammals corresponds with the degree of development of 

 that sense and is markedly reduced in man. The development of the 

 cerebral hemispheres even in lower mammals is such that important 



Fig. 42. The brain from the left side: bo, olfactory bulb; c, cerebellum; 

 fc, paraflocculus cerebelli; h, cerebral hemisphere; Ip, piriform lobe; m, 

 medulla oblongata; s, spinal cord. Numerals _ indicate the corresponding 

 cranial nerves: 2, optic; 4, trochlear; 5, trigeminal; 6, abducens; 7, facial; 

 8, acoustic; 9, glossopharyngeal; 10, vagus; 11, spinal accessory; 12, 

 hypoglossal. 



parts of the brain, notably the diencephalon and mesencephalon 

 (pp. 80, 81), are concealed dorsally and laterally. However, the 

 chiasma or crossing of the optic nerves on the ventral aspect of the 

 diencephalon is evident, while more posteriorly the convergent cords 

 of the cerebral peduncles may be seen passing backwards on that 

 of the mesencephalon. The posterior part of the brain is formed 

 largely by the cerebellum above, the corrugation of which is one 



