DEVELOPMENT OF THE NERVOUS SYSTEM. 771 



quadrigemina (Fig. 284) ; the cerebellum tending in the same way to 

 grow backward, and projecting farther in this direction over the medulla 

 oblongata. The subsequent history of the development of the enceph- 

 alon is mainly a continuation of the same process ; the relative dimen- 

 sions of the parts constantly changing, so that the hemispheres become, 

 in the adult condition (Fig. 285), the largest division of the encephalon. 



Fig. 285. 



BRAIN OF ADULT PIG. 1. Hemispheres. 3. Cerebellum. 4. Medulla oblongata. 



while the cerebellum is next in size, and covers the upper portion of 

 the medulla oblongata. The surfaces of the hemispheres and cerebellum, 

 which are at first smooth, become afterward convoluted ; thus increasing 

 still farther the extent of their nervous matter. In the human foetus 

 the cerebral convolutions begin to appear about the beginning of the 

 fifth month (Longet), and grow deeper and more abundant during the 

 remainder of foetal life. 



The lateral portions of the brain growing at the same time more 

 rapidly than that on the median line, they project on each side outward 

 and upward ; and by folding over against each other toward the median 

 line, they form the right and left hemispheres, separated by the longi- 

 tudinal fissure. A similar process of growth in the spinal cord results 

 in the formation of its two lateral halves, and the anterior and posterior 

 median fissures of the cord. Elsewhere the median fissure is less com- 

 plete, as, for example, between the two lateral halves of the cerebellum 

 or those of the medulla oblongata ; but it exists everywhere, and marks 

 more or less distinctly the division between the two sides of the nervous 

 centres, produced by the excessive growth of their lateral portions. In 

 this way the whole cerebro-spinal axis is converted into a double organ, 

 equally developed upon the right and left sides, and partially divided 

 by longitudinal median fissures. 



Organs of Special Sense. The eyes are formed by a diverticulum 

 which grows out on ench side from the first cerebral vesicle. This 

 diverticulum is at first hollow, its cavity communicating with that of 

 the hemisphere. Afterward, the passage between the two is filled with 

 a deposit of nervous matter, and becomes the optic nerve. The globular 

 portion of the diverticulnm, which is converted into the globe of the 

 eye, has a thin layer of nervous matter deposited upon its internal sur- 



