FARTHER DEVELOPMENT OF THE EXCEPHALON. 



757 



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 ccrchval jwdunclcs are increasing rapidly in size in the floor of this 



Fig. 557. — Brain of the Human Em- Fig. 557. 



BRYO OF THREE MONTHS. NATURAL /, ^ 



SIZE (from KoUiker). 



In 1 the view is from above, the 

 Upper fiart of the cerebral hemispheres 

 and mesencephalon having been re- 

 moved. /, fore-part of the divided 

 wall of the hemisphere ; /', hind part 

 of the same turned in which becomes 

 the hippocampus ; est, corpus striatum ; 

 tha, thalamus opticus. 



In 2 the lower surface is represented \ 

 to, ti'actus opticus ; and in front of 



this the olfactory bulbs and tracts ; c m, single mass of the corpora mammillaria not yet 

 divided ; p, pons Varolii. The cerebellum and medulla oblongata are seen behind and 

 to the sides in both iigures. 



(A a. 



middle cephalic vesicle, the cavity in its interior is quickly filled up, with the 

 exception of the narrow passage named the SijJrian aqueduct. The fillet is dis- 



Fig.. 558. — Brain and Spinal Cord op a F(etus of four Fig. .558 



MONTHS, seen from BEHIND (from Kolliker). 



h, hemispheres of the cerebrum ; m, corpora quadrigemina 

 or mesencephalon ; f, cerebellum ; m o, medulla oblongata, the 

 fourth ventricle being overlapped by the cerebellum ; s s, the 

 spinal cord with its brachial and crural enlargements. 



tmguishalble in the fourth month. The corpora quad- 

 rigemma of the two sides are not marked off from 

 each other by a vertical median groove until about the 

 sixth month ; and the transverse depression separating 

 the anterior and posterior pau-s is first seen about the 

 seventh month of intra-uterine life. 



Thalamencephalon. — From the sides of this vesicle, 

 as has already been described, the optic vesicles are fonned. 

 and from its forepart on the two sides the vesicles of 

 the cerebral hemispheres are developed. Reichert first 

 pointed out that there is left between the hemu2)?ier(- 

 Tcslclcs of opposite sides a wedge-shaped interval, which 

 forms the thu-d ventricle. He points out that the 

 tenninal extremity (lamina terminalis) of the cerebro- 

 spinal tube is at the tip of this wedge, and is placed im- 

 mediately in front of the optic commissure, at the lamina 

 cinerea ; and that therefore the infundibulum is not that 

 extremity, as had been previously supposed by Baer, but 

 is an expansion of the vesicle downwards.- 



The formation of the pituitary body has akeady been 

 described. The infundibulum of the thalamencephalon 



becomes connected with it superiorly, and seems for a time even to form a part 

 of it. 



The pineal gland, according to Baer, is developed from the back part of the 

 thalami, where those bodies continue joined together ; but it is suggested by 

 Bischoff that its development may be rather connected with the pia mater. It 

 was not seen by Tiedemann until the fourth month in the human foetus ; subse- 

 quently its growth is very slow : and it at first contains no gritty deposit : this, 

 however, was found by Sommeiiing at birth. 



