DEVELOPMENT OF THE NERVOUS SYSTEM. 669 



of the encephalon, while the cerebellum is next in size, and covers the 

 upper portion of the medulla oblongata. The hemispheres and cere- 

 bellum, 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 fifth 

 month (Longet) ; growing deeper and more abundant during the 

 remainder of foetal life 



The lateral portions of the brain, enlarging at the same time more 

 rapidly, project on each side outward and upward ; and coming in con- 

 tact with each other along the median line, form the right and left 

 hemispheres, separated by the longitudinal fissure. A similar growth 

 in the spinal cord results in the formation of its two lateral halves, 

 separated by the anterior and 

 posterior median fissures. 

 Elsewhere the median fissure 

 is less complete, as, for exam- 

 ple, between the two lateral 

 halves of the cerebellum a:.d 

 medulla oblongata ; but it ex- 

 ists everywhere, and marks 

 more or less distinctly the 

 division of the nervous cen- 

 tres, produced by the growth 



Of their lateral parts. In this BRAI * OF A f VLT * K -~ Hemispheres. 3. Cerebel- 



lum. 4. Medulla oblongata. 



way the whole cerebro-spinal 



axis is converted into a double organ, equally developed on the right 



and left sides, and partially divided by longitudinal median fissures. 



Organs of Special Sense. The eye is formed on each side by a lat- 

 eral offshoot from the first cerebral vesicle. It is at first a hollow 

 diverticulum, the cavity of which communicates with that of the hemi- 

 sphere from which it was produced. Afterward, the passage of com- 

 munication is filled with a deposit of nervous matter, which becomes the 

 optic nerve. The globular portion of the diverticulum, which is con- 

 verted into the ey ball, is lined posteriorly by a thin layer of nervous 

 matter, which becomes the retina ; its cavity being occupied by a gela- 

 tinous substance, the vitreous body. The crystalline lens is formed in 

 a distinct follicle, an offshoot of the integument, which becomes par- 

 tially imbedded in the anterior portion of the eyeball. The cornea is 

 also originally a part of the integument, and remains somewhat opaque 

 until a late period of development. It becomes nearly transparent a 

 short time before birth. 



The iris is a muscular septum, formed in front of the crystalline 

 lens. Its central opening, which afterward becomes the pupil, is at 

 first closed by a vascular membrane the pupillary membrane, pass- 

 ing across the longitudinal axis of the eye. The blood-vessels of this 

 membrane, which are derived from those of the iris, subsequently be- 

 come atrophied. They first disappear from its centre, and recede grad- 



