FORMATION OF THE EXCEPHALOX. 

 THE EXCEPHALOX. 



575 



The brain is originally not to be distinguished from the spinal cord, being in fact 

 the anterior portion of the medullary tube. It is soon altered in form, however, by 

 the expansion of its walls in certain parts, while others enlarge in a less degree, and it 

 then presents the appearance of a series of three cerebral vesicles, usually designated 

 by embryologists the primary cerebral vesicles. 



Fig. 389. SKETCHES OF THE PRIMITIVE PARTS 

 OF THE HUMAN BRAIN (from Kolliker). 



Fig. 389. 



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

 about seven weeks. 1, view of the whole 

 embryo from behind, the brain and spinal cord 

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

 view of the brain removed from the body ; A, 

 the anterior part of the first primary vesicle 

 or cerebral hemisphere (prosencephalon) ; z, 

 the posterior part of the same vesicle (dien- 

 cephalon) ; i', the lower part of the same ; m, 

 the middle primary vesicle (inesencephalon) ; e, 

 the cerebellum or upper part of the third 

 primary vesicle (epencephalon) ; m o, the lower 

 part of the third primary vesicle or medulla 

 oblongata. The figure 3 illustrates the several 

 curves which take place in the development 

 of the parts from the primitive medullary tube. In 4, a lateral vie* is given of the 

 brain of a human embryo of three months : the enlargement of the cerebral hemisphere 

 has covered in the optic thalami, leaving the tubercula quadrigemina, m, apparent. 



The changes which take place in the growth of the brain were first elaborately 

 described by Tiedemann ; they have been investigated by Von Baer, Bischoff, Remak, 

 Kolliker, and others, and have recently received additional elucidation from the 

 researches of Reichert. (Tiedemann, " Anatomic und Bildungsgeschichte des 

 Gehirns," Nuremberg, 1816 ; Reichert, " Bail des Menschlichen Gehirns," Leipzig. 

 1861 ; F. Schmidt, " Beitrage z. Entwick. des Gehirns," in Zeitschr. f. Wissen. Zool. 

 1862 ; Kolliker, Entwicklungsgeschichte, 1861.) 



DEVELOPMENT OP THE PRIMARY VESICLES. The anterior or first vesicle, is the part 

 from which are developed the third ventricle, the optic thalami, the corpora striata, 

 and the cerebral hemispheres. 



The middle or second vesicle, forms the corpora quadrigemina above, and the crura 

 cerebri below, its cavity remaining as the Sylvian aqueduct. 



The posterior or third vesicle, continues incomplete above for some time, in so far as 

 relates to its nervous substance. At length its anterior portion is closed over and forms 

 the cerebellum above, whilst in its under part the pons Varolii is produced. The 

 posterior portion, on the other hand, continues open on its dorsal aspect, and forms the 

 medulla oblongata and fourth ventricle. 



Fig. 390. LONGITUDINAL SECTION OF THE 

 CRANIAL CAVITY OF THE HUMAN EMBRYO AT 

 FOUR WEEKS (from Kolliker). 



1, 2, 3, 4, and .*>, mark the depressions in 

 the cranial wall which contain respectively the 

 cerebral hemispheres, the thalarai, the corpora 

 quadrigemina, the cerebellum, an,l the medulla 

 oblongata; in 1, at o, the depression of the eye, 

 and at o', the optic nerve is seen ; in 5, at a u, 

 the primary auditory vesicle ; p, process from 

 the phai'ynx, supposed by llathke to be con- 

 nected with the formation of the pituitary 

 body or hypophysis cerebri ; t, t, middle 

 cranial septum or teutorium. 



These three vesicles, at first arranged in a straight line, one before the other, soon 



