io6a 



HUMAN ANATOMY. 



Telencephalon 



Corpus striatum 



Optic recess 



Mesencephalon 



Isthmus 



Metencephalon 



Myelencephalon 



above the anterior end of the primary gut-tube and of the notochord. At first the 

 axis ot the fore-brain lies about at right angles with that of the rhombencephalon, 



(Fig. 911) but, with the in- 



FIG. 913. creasing size of the middle 



an( j anterior vesicles, the 

 angle of the flexure becomes 

 more acute until the long 

 axis of the fore-brain and 

 of the rhombencephalon are 

 almost parallel (Fig. 912). 

 During the fourth week 

 a second ventral bend, the 

 cervical flexure, appears 

 at the lower end of the hind- 

 brain and marks the separa- 

 tion of the encephalic from 

 the spinal portion of the 

 neural tube. The cervical 

 flexure, which also involves 

 the head, is most evident 

 at the close of the fourth 

 week, when it is almost a 

 right angle ( Fig. 912); after 

 this it becomes less pronounced in consequence of the elevation of the head which 

 succeeds the period when the embryonic axis is most bent. 



The third flexure appears about the fifth week in the part of the metencephalon 

 in which the pons is later developed and, hence, is termed the pontile flexure. 

 It concerns chiefly the ventral wall, which is in consequence for a time ventrally 

 doubled on itself ; subsequently this flexure almost entirely disappears. In contrast 

 to the preceding bends, this flexure is only partial and involves chiefly the ventral 

 and only slightly the dorsal wall of the neural tube ; on the exterior of the embryo its 

 presence is not detectable. 



The developmental relations of the chief parts of the fully formed brain to the 

 embryonic brain-vesicles are shown in the accompanying table. 



TABLE SHOWING RELATIONS OF BRAIN-VESICLES AND THEIR DERIVATIVES. 



\ 



Ventral Dorsal 



zone of brain-wall 



Diagram showing five cerebral vesicles and dorsal and ventral zones of 

 their wall ; based on brain of embryo of four and one-half weeks. (//.) 



Notwithstanding the great changes in position and relation which many parts of 

 the human brain suffer during development, chiefly in consequence of the enormous 

 expansion of the pallium and the correspondingly large size of its commissure, the 



