PARTS DERIVED FROM PRIMARY CEREBRAL VESICLES. 577 



subsisting between the primary encephalic vesicles, the five fundamental parts, and 

 the principal permanent structures of the brain : 



( Cerebral Hemispheres, Corpus Callosum, 



/I. Prosencephalon. * I Corpora Striata, Fornix, Lateral Yen- 

 I. Anterior primary Vesicle J ' tricles ' Olfa 'tory nerve. 



U Diencephalon. 



II. Middle primary Vesicle, 3. Mesencephalon. 



. ,-, , , ( Cerebellum, Pons Varolii, anterior part of 



f 4 - Epencephalon. J th(J Fom ^ Ventricle> 



III. Posterior primary Vesicle, < 



I , ,, . , . ( Medulla Oblongata, Fourth Ventricle, Au- 



lo. Metencephalon. { ditory nerve * 



At a later period of development, the anterior part of the first vesicle, which, as 

 stated above, represents the cerebral hemispheres, increases greatly in size upwards 

 and backwards, and gradually covers the parts situated behind it ; first the thalami, 

 then the corpora quadrigemina, and lastly the cerebellum. 



On laying open the rudimentary encephalon, two tracts of nervous matter are seen 

 to be prolonged upwards from the spinal cord upon the floor of the cephalic vesicles ; 

 these tracts, which are doubtless connected with the anterior and lateral parts of the 

 cord, are the rudiments of the crura cerebri and corresponding columns of the 

 medulla oblongata. 



FARTHER DEVELOPMENT OF THE PRIMARY VESICLES. The third vesicle. The poste- 

 rior portion of this vesicle, corresponding with the medulla oblongata, is never closed 

 above by nervous matter. The open part of the medullary tube constitutes the'floor 

 of. the fourth ventricle, which communicates below with the canal of the spinal cord, 

 at the place where the calamus scriptorius is eventually formed. 



The three constituent parts of the medulla oblongata begin to be distinguished about 

 the third month ; first the restiform bodies, which are connected with the commencing 

 cerebellum, and afterwards the anterior pyramids and olives. The anterior pyramids 

 become prominent on the surface and distinctly denned in the fifth month ; and by 

 this time also their decussation is evident. The olivary fasciculi are early distin- 

 guishable, but the proper olivary body, or tubercle, does not appear till about the 

 sixth month. The fasciolae, cinerece of the fourth ventricle can be seen at the fourth 

 or fifth month, but the white strice not until after birth. 



The anterior part of the third vesicle is soon closed above by nervous substance, 

 and forms the commencing cerebellum. This part exists about the end of the second 

 month, as a delicate medullary lamina, forming an arch behind the corpora quadri- 

 gemina across the widely open primitive medullary tube. 



According to Bischoff, the cerebellum does not commence, as was previously sup- 

 posed, by two lateral plates which grow up and meet each other in the middle line ; but 

 a continuous deposit of nervous substance takes place across this part of the medullary 

 tube, and closes it in at once. This layer of nervous matter, which is soon connected 

 with the corpora restiformia, or inferior peduncles, increases gradually up to the fourth 

 month, at which time there may be seen on its under surface the commencing corpus 

 dentatum. In the fifth month a division into five lobes has taken place ; at the sixth, 

 these lobes send out folia, which are at first simple, but afterwards become subdivided. 

 Moreover, the hemispheres of the cerebellum are now relatively larger than its 

 median portion, or worm. In the seventh month the organ is more complete, 

 and the flocculus and posterior velum, with the other parts of the inferior vermiform 

 process, are now distinguishable, except the amygdala, which are later in their 

 appearance. 



Of the peduncles of the cerebellum, the inferior pair (corpora restiformia) are the 

 first seen viz., about the third month ; the middle peduncles are perceptible in the 

 fourth month ; and at the fifth, the superior peduncles and the Vieussenian valve. 



* This and the four following terms are adopted as applicable to the principal secondary 

 divisions of the primordial medullary tube, and as corresponding to the commonly received 

 names of the German embryologists, viz., Vorderhirn, Zwischenhirn, Mittelhirn, Hinter- 

 hirn, and Nachhirn ; or their less used English translations, viz., forebrain, interbrain, 

 midbrain, hindbrain, and afterbrain. 



