754 



DEYELOrMENT OF THE NEHVOUS SYSTEM. 



of very simple modifications of the primitive medullary tube, it is mainly 

 by an increased thickening of the medullary wall in some of the parts, 

 and the relative thinning, or even the removal of the substance in 

 others, that the changes accompanying the formation of the cerebral 

 masses are effected, while as a consequence of these and other modifi- 

 cations of form, the several parts of the internal cavity or ventricles of 

 the brain acquire the different degrees of expansion and contraction, or 

 the comparatively closed or open condition which they exhibit in after 

 life. Thus the cerebral hemispheres, and corpora striata are the main 

 masses formed by the lateral thickening and expansion of the medullary 

 walls of the procerebrum, while the corpus callosum and fornix are 

 formed later by a deeper median development in connection with these 

 parts : the thalami optici are the most solid parts of the lower and 

 lateral region of the second rudiment, while in the upper wall the 

 pineal gland, and in the lower the infundibulum with the hypophysis 

 cerebri, are added : the corpora quadrigemina are thickenings of the 

 upper wall of the third rudiment, while the crura cerebri arise by 

 increased deposit in its lower part ; the cerebellum is a large deposit 

 in the upper wall of the fourth radiment, while the pons Varolii is a 

 thickening of its lower wall ; and the parts composing the medulla 

 oblongata are principally formed by increased deposit in the lower and 

 lateral wall of the fifth rudiment. 



Thus, also, the lateral ventricles are two lateral expansions of the 

 forepart of the original ventricular cavity which follow the dilatation of 

 the vesicles of the right and left cerebral hemispheres, and communi- 

 cate with the central or third ventricle by the common foramen of 

 Monro. The central or third ventricle, originally the foremost part of 

 the medullary hollow, is narrowed on the sides by the increased develop- 

 ment of the thalami optici, while inferiorly it is prolonged and projects 

 downwards as infundibulum into the pituitary fossa ; and above the 

 wall of this ventricle comes to be opened up by the thinning aAvay of 

 its medullary substance, excepting at the place where it is crossed by 

 the pineal gland. The continuation backwards of the original ventri- 

 cular hollow, greatly narrowed by the ultimate thickening of the sub- 

 stance of the corpora quadrigemina and crura cerebri, forms the aque- 



Fis. 555. 



Fig. 555. — Vertical Section of thr Brain of a 

 Human Embryo of fourteen weeks, Magnified 

 THREE Diameters (from Reicliert). 



c, cerebral bemispliere ; cc, carpus callosum be- 

 beginning to pass back , /, foramen of Monro ; p, 

 membrane over tbe third ventricle and the pineal 

 gland ; tk, thalamus opticus ; 3, third ventricle ; 

 I, olfactory bulb ; cq, corpora quadrigemina, mesen- 

 cephalon : cr, crura cerebri, and above them the 

 aqueduct of Sylvius still wide ; c', cerebellum, and 

 below it the fourth ventricle ; _/)c, Pons Varolii ; 

 m, medulla oblongata. 



dnct of Sylvius, or the iter a tertio ad quartura ventriculmn, and is 

 succeeded by the more expanded cavity of the fourth ventricle, lying 

 between the cerebellum and the lower wall. The upper wall of the latter 

 cavity undergoes great thinning, like that of the third ventricle, so as 

 to be reduced m the part before the cerebellum to the valve of Vieussens, 



