600 NERVOUS SYSTEM. 



The longitudinal commissural fibres connect together distant parts of the same 

 hemisphere, the fibres being disposed in a longitudinal direction. They form 

 the fornix, the tsenia semicircularis, and peduncles of the pineal gland, the striae 

 longitudinales, the fibres of the gyrus fornicatus, and the fasciculus unciformis. 



THE CEREBELLUM. 



The Cerebellum, or little brain, is that portion of 'the encephalon which is 

 contained in the inferior occipital fossae. It is situated beneath the posterior 

 lobes of the cerebrum, from which it is separated by the tentorium. Its average 

 weight in the male is 5 oz. 4 drs. It attains its maximum weight between the 

 twenty-fifth and fortieth year; its increase in weight after the fourteenth year 

 being relatively greater in the female than in the male. The proportion 

 between the cerebellum and cerebrum is, in the male, as 1 to 8$ ; and in the- 

 female, as 1 to 8J. In the infant, the cerebellum is proportionally much smaller 

 than in the adult, the relation between it and the cerebrum being, according to 

 Chaussier, between 1 to 13, and 1 to 26; by Cruveilhier the proportion was 

 found to be 1 to 20. In form, the cerebellum is oblong, and flattened from 

 above downwards, its greatest diameter being from side to side. It measures 

 from three and a half to four inches transversely, and from two to two and a 

 half inches from before backwards, being about two inches thick in the centre, 

 and about six lines at the circumference, which is the thinnest part. It consists 

 of gray and white matter: the former, darker than that of the cerebrum, occu- 

 pies the surface; the latter the interior. The surface of the cerebellum is not 

 convoluted like the cerebrum, but traversed by numerous curved furrows or 

 sulci, which vary in depth at different parts, and separate the laminae of which 

 its exterior is composed. 



Its upper surface (Fig. 334) is somewhat elevated in the median line, and 

 depressed towards its circumference; it consists of two lateral hemispheres, con- 

 nected together by an elevated median portion or lobe, the superior vermiform 

 process. The median lobe is the fundamental part, and in some animals, as 



Fig. 334. Upper Surface of the Cerebellum. 



fishes and reptiles, the only part which exists; the hemispheres being additions, 

 and attaining their maximum size in man. The hemispheres are separated, in 

 front, by a deep notch, the incisura cerebelli anterior, which encircles the corpora 

 quadrigemina behind; they are also separated by a similar notch behind, the 

 incisura cerebelli posterior, in which is received the upper part of the falx cere- 

 belli. The superior vermiform process (upper part of the median lobe of the 

 cerebellum) extends from the notch on the anterior to that on the posterior 



