CHAPTER X 



THE CEREBRUM 



THE cerebrum includes all of the brain lying in front of the 

 isthmus, that is, the midbrain (mesencephalon), betweenbrain 

 (diencephalon), and cerebral hemispheres (telencephalon), the 

 two last comprising the forebrain (prosencephalon). It con- 

 tains the primary sensory centers of the olfactory nerves (I 

 pair), the sensory correlation centers of smell and sight, the 

 primary motor and sensory centers of the oculomotor and troch- 

 lear nerves (III and IV pairs) for movements of the eyes, and all 

 of the most important higher correlation centers of the brain. 

 These higher correlation centers make up by far the larger part 

 of its substance in the human brain, though in fishes the converse 

 relation prevails, with the primary sensori-motor centers and the 

 simpler correlation mechanisms making up the larger part (see 

 Figs. 43, 44, pp. Ill, 112). 



The mesencephalon (midbrain) is that part of the brain in 

 which the early embryonic neural tube (Figs. 46-51, pp. 116- 

 119) has been least modified in the adult. The ventral part 

 of the midbrain, i. e., the part lying ventrally of the ventricle, 

 which is here termed the aqueduct of Sylvius, is called the cere- 

 bral peduncle; the dorsal part is the corpora quadrigemina, the 

 upper pair of these four eminences being the superior colliculi, 

 and the lower pair the inferior colliculi (see Fig. 71, p. 154). 



The corpora quadrigemina contain important correlation 

 centers, the superior colliculus chiefly visual (p. 209) and the 

 inferior colliculus chiefly auditory (p. 202). The cerebral 

 peduncle, as the name implies, contains the great ascending and 

 descending fiber tracts between the forebrain above and the 

 medulla oblongata, cerebellum, and spinal cord below. The 

 arrangement of some of these tracts can be seen in Fig. 75. The 

 cerebral peduncle also contains the nuclei of origin for the motor 

 fibers of the III and IV pairs of cranial nerves and several masses 



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