578 



DEVELOPMENT OF THE CEREBRO-SPINAL AXIS. 



The pons Varolii is formed, as it were, by the fibres from the hemispheres of the 

 cerebellum embracing the pyramidal and olivary fasciculi of the medulla oblongata. 

 According to Baer, the bend which takes place at this part of the encephalon 

 thrusts down a mass of nervous substance before any fibres can be seen ; and in 

 this substance transverse fibres, continuous with those of the cerebellum, are after- 

 wards developed. From its relation to the cerebellar hemispheres the pon.s keeps 

 pace with them in its growth ; and, in conformity with this relation, its transverse 

 fibres are few, or entirely wanting, in those animals in which there is a corresponding 

 deficiency or absence of the lateral parts of the cerebellum. 



Fig. 392. Fig. 392 A. BRAIN AND SPINAL CORD EXPOSED FROM BEHIND IN 



^ A FOSTUS OF THREE MONTHS (from Kolliker). 



h, the hemispheres ; m, the mesencephalic vesicle or corpora 

 quadrigemina ; c, the cerebellum ; below this are the medulla 

 oblongata, mo, and fourth ventricle, with remains of the membrana 

 obturatoria. The spinal cord, s, extends to the lower end of the 

 sacral canal and presents the brachial and crural enlargements. 



Fig. 392 B. UPPER VIEW OF THE BRAIN OF A THREE MONTHS' 



FffiTUS, IN WHICH THE HEMISPHERES HAVE BEEN DIVIDED AND 

 TURNED ASIDE, AND THE VESICLE OF THE MESENCEPHALON (CoR- 



PORA QDABRIGEMINA) OPENED (from Kolliker). 



/, anterior part of the great arch of the hemispheres over the 

 cerebral fissure ; f, posterior part descending into the cornu am- 

 monis ; cs, corpus striatum ; th, thalaraus opticus ; m, in the 

 floor of the opened vesicle of the mesencephalon, which is still 

 hollow. 



The second, or middle vesicle. The corpora quadrigemina 

 are formed in the upper part of the middle cephalic vesicle ; 

 the hollow in the interior of which communicates with those 

 of the first and third vesicles. The corpora quadrigemina, in 

 the early condition of the human embryo, are of great propor- 

 tionate volume, in harmony with what is seen in the lower 

 vertebrata : but subsequently they do not grow so fast as the 

 anterior parts of the encephalon, and are therefore soon 

 overlaid by the cerebral hemispheres, which at the sixth month 

 cover them in completely. Moreover, they become gradually 

 solid by the deposition of matter within them ; and as, in the 

 meantime, the cerebral peduncles are increasing rapidly in size 

 in the floor of this middle cephalic vesicle, the cavity in its 

 interior is quickly filled up, with the exception of the narrow 

 passage named the Sylvian aqueduct. The fillet is distinguish- 

 able in the fourth month. The corpora quadrigemina of the 

 two sides are not marked off from each other by a vertical 

 median groove until about the sixth month ; and the trans- 

 verse depression separating the anterior and posterior pairs' 

 is first seen about the seventh month of intra-uterine life. 



The first, or anterior vesicle. This vesicle, very soon after its formation, exhibits 

 two lateral outgrowths the optic vesicles, destined to form the fundamental parts of 

 the organs of vision. Each of these soon becomes separated from the parent vesicle 

 by a constricted part, which forms the optic nerve and tract. The first vesicle has 

 usually been described as dividing into two portions viz., a posterior, which is 

 developed into the optic thalami and third ventricle, and an anterior, which forms the 

 principal mass of the cerebral hemispheres, including the corpora striata. Reichert, 

 however, has pointed out that the hemispheres and corpora striata are developed 

 from the sides of the fore part of the vesicle, and become distinguished from it by a 

 constriction similarly as the optic vesicles had previously been, and that there is left 

 between the hemisphere-vesicles of opposite sides a wedge-shaped interval, which forms 

 the third ventricle. He points out that the terminal extremity of the cerebro-spinal 

 tube is at the tip of this wedge, and is placed immediately in front of the optic 

 commissure, at the lamina cinerea ; and that therefore the infundibulum is not that 



