752 DEVELOPMENT OF THE XERVOUS SYSTEM. 



form of this re,e,'ion of the head has already been adverted to, aud need 

 not be repeated here. 



As regards the earh'est phenomena of development in the brain itself, 

 there are three changes which mainly tend to modify its form in the 

 most marked degree, viz., 1st, the development from the anterior vesicle 

 on each side of the primitive ocnlar vesicle ; 2nd, the expansion from 

 another part, somewliat later, of the vesicles of the cerebral hemis- 

 pheres; and 3rd, tlie formation in the forepart of the posterior encephalic 

 vesicle of a new cerebral rudiment corresponding to the cerebellun). 



Fig. r)53. Fig. 553. — Sketches op the Peimitiye Parts 



I „T ., OF THE Human Brain (from Kolliker). 



1, 2, and 3 are from the human embryo of 

 about seven weeks. 1, view of the wliole 

 embryo from behind, the brain and spinal 

 cord exposed ; 2, the posterior, and 3, the 

 lateral view of the brain removed from the 

 body ; h, the cerebral hemisj)here (prosen- 

 cephalon) ; i, the thalamenceiihalou ; i', the 

 infundibulum at the lower part of the same ; 

 m, the middle ^jrimary vesicle (mesen- 

 cephalon) ; c, the cerebellum (epencephalou) ; 

 m 0, the medulla oblongata. Figure 3 shows 

 also the several curves which take place in 

 the development of the parts from the primi- 

 tive medullary tube. In 4, a lateral view is 

 given of the brain of a human embrjo of 

 three months : the enlargement of the cerebral 



hemisphere has covered in the optic thalami, leaving the tubercula quadrigemina, ?;?, 



apparent. 



The formation of the primitive ocnlar vesicles, by an evolution of the 

 lateral wall of the primitive medullary tube, gives to the first vesicle and 

 the adjacent part of the head a much greater lateral width ; but the 

 cranial wall, though pushed out by the enlarging oculai: vesicles, does 

 not follow closely the inflection of their surfaces. As the subse- 

 quent contraction of the stalk of the ocular vesicles progresses, these 

 vesicles are thrown more backwards and downwards by the change 

 next to be described. 



Tills is the evolution or expansion of the wall of the anterior ence- 

 phalic vesicle into the two cerebral hemispheres, which takes place in 

 front and at each side, so that the vesicles of the right and left hemis- 

 pheres are from the first separate and distinct. As these vesicles 

 become dilated, the cranial wall undergoes a corresponding expansion 

 in the forepart of the head, and the vesicle of the thalamencephalon, 

 which was at first the foremost part of the embryo-head, is thrown 

 downwards and backwards into a deeper position. 



The middle encephalic vesicle, increasing greatly in size, takes the 

 most prominent part of the head superiorly, both from its own greatei 

 relative magnitude, and from the sudden bend which the head now 

 takes below this vesicle in the great cranial curvature. 



The formation of the cerebellum begins by a thickening in the upper 

 and lateral walls of the part of the posterior primitive vesicle which is 

 next to the midbrain, and is accompanied by a deep inflection of the 

 medullary tube between it and the remaining part of the vesicle which 

 forms the medulla oblongata. 



