NERVOUS SYSTEM AIJTD GENERAL SENSATION. 41 



in the cerebrum of the higher orders of mammals, whilst in 

 some of the lower orders, as the rodentia (figs. 28 and 29), 

 they entirely disappear. The inferior surface of the cerebrum is 

 divisible into three lobes, separated from each other by trans- 

 verse furrows (fig. 20). a is the anterior, b the middle, c the 

 posterior lobes. Near the median line we observe two round 

 eminences, the optic lobes, g ; and two large masses of neurine, 

 the peduncles of the brain, which pass downwards to be con- 

 tinued into the medulla oblonyata. It is from the base of the 

 brain, likewise, that the nerves proceed which are classed under 

 the division cerebral. The surface of the cerebrum is formed 

 almost entirely of grey nervous substance, which covers the in- 

 ternal white neurine. When we cut off the hemispheres parallel 

 to the corpus callosum, we observe that the cerebrum contains 

 internally several cavities communicating with each other, 

 called the ventricles of the brain. In these chambers several 

 bodies are found, the study of which more especially belongs 

 to the professed anatomist. 



[ 87. The CEREBELLUM occupies the posterior and inferior 

 part of the skull (fig. 19, b. fig. 20, d) : its weight, as com- 

 pared with that of the brain, is, in man, 1 : 9, whilst in other 

 mammals it varies from 1 : 2 to 1 : 14. It is protected from the 

 pressure of the posterior lobes of the cerebrum by a large ex- 

 tension of the dura mater (tentorium cerebelli), which becomes 

 an osseous plate in the carnivora. The cerebellum is divided 

 into two large lateral lobes, and one small central lobe. The 

 lateral lobes are separated by a membranous process (falx 

 cerebelli), and the middle lobe is situated in a depression be- 

 hind and below them. In the quadrumana (figs. 32 and 33), 

 the third lobe is proportionally larger ; and in the rodentia 

 (figs. 28 and 29) it equals in volume a lateral lobe. The 

 nervous substance is folded into a series of transverse con- 

 centric lamellae, placed perpendicularly on their edges, and 

 enclosed one within the other. If the sulci are carefully 

 opened, several other lamellae will be found enclosed within 

 them, but smaller in size, more irregular, and with various 

 degrees of inclination. The distribution of the neurine is 

 seen on making a vertical section of one of the lateral lobes, 

 as shown at (d) figure 20. The white substaatce is found so 

 disposed as to resemble the stem and branches of a tree, and 

 hence called the arbor vitce. The branches project into the 

 lamellae, and are invested with a covering of grey substance. 



