66 



CEREBELLAR VESICLE. 



the earliest appearance of the nerve roots, the white bundles not yet medullated, how- 

 ever which are known as the ascending root of the fifth, and the ascending- root of the va<?us 

 and glossopharyngeal (solitary bundle) begin to make their appearance, both being at first on 

 the surface of the medulla. They gradually, however, become covered in by a folding over of 

 the dorsal part of the alar lamina, and thus come later to lie imbedded in the substance of 

 each lateral half of the medulla. This fold is shown in its commencement in fig. 73, A and 

 B, /. According to His the bundles grow downwards towards the spinal cord from the places 

 of entrance of the corresponding nerve roots, emerging from the ganglia, as in the case of the 

 posterior spinal roots ; and after entering the medulla grow gradually along the course of the 

 future so-called ascending roots, so that the latter are at first visible only in sections taken 

 near the places of entrance of the nerve roots into the medulla. 



The fourth cerebral vesicle: cerebellar vesicle, or epeucephalou. The 



constriction, which is at first obvious between this and the fifth vesicle, does not 

 long persist, so that the two together form a long boat-shaped cavity which becomes 



Fig. 74. MEDIAN SECTION THROUGH 



THE BRAIN OF A TWO AND A 

 HALF MONTHS FCETUS. (HlS.) 



Magnified 5 diameters. 



The mesial surface of the left 

 cerebral hemisphere is seen in the 

 upper and right hand part of the 

 figure ; the large cavity of the third 

 ventricle is bounded above and in 

 front by a thin lamina ; below is seen 

 the infundibulum and pituitary body. 

 Filling the upper part of the cavity 

 is the thalamus opticus ; in front and 

 below this is the slit-like foramen of 

 Monro. Behind the thalamus is seen 

 another slit-like opening which leads 

 into the still hollow external geni- 

 culate body, 



o'f, olfactory lobe ; p, pituitary 

 body ; c q , corpora quadrigemina ; 

 c 1 ), cerebellum ; m.o., medulla ob- 

 longata. 



\ \ the fourth ventricle. As in 



that part of this cavity which 



has already been described with the fifth vesicle, the roof inferiorly becomes greatly 

 thinned and expanded. Superiorly the tube becomes gradually more contracted and 

 the roof thicker, this thickening being the rudiment of the cerebellum and of the 

 valve of Vieussens (fig. 74). In the meanwhile a considerable thickening of the 

 lateral boundaries, which, as in the medulla oblongata, have been thrown outwards 

 by the roof expansion, occurs, and from this the substance of the pom is gradually 

 formed. 



The dorsal and ventral laminae of the lateral walls are still evident in this part of 

 the embryonic brain. With the former, the sensory fibres of the fifth nerve are 

 immediately connected ; with the latter, the motor fibres of the fifth and also the 

 sixth and seventh nerves. 



In the human embryo the cerebellum is seen as early as the second month, 

 forming a thin plate arching over the anterior part of this vesicle (fig. 74). From 

 this plate, which enlarges only gradually, is formed the middle lobe ; later the lateral 

 lobes grow out at the sides. The cerebellar surface is at first smooth, but a sub- 

 division into the subordinate lobes occurs in the fifth month, and the folia appeal- 

 about the sixth. In the seventh month all the parts of the organ, except the 

 amygdalifi, are formed. 



Of the cerebellar peduncles, the inferior appear in the third month, the middle in 

 the fourth, and the superior in the fifth. The transverse fibres of the pons develop 

 part passu with the lateral lobes, appearing about the fourth month. 



olf 



