DEVELOPl 



appears in the region of the mid-brain, and is named the ventral cephalic flexure 

 (Fig. 650). By means of it the fore-brain is bent in a ventral direction around 

 the anterior end of the notochord and fore-gut, with the result that the floor of 



the fore-brain comes to lie almost parallel with that of the hind-brain. This 

 flexure causes the mid-brain to become, for a time, the most prominent part of 

 the brain, since its dorsal surface corresponds with the convexity of the curve. 



Auditory vesicle 

 Facial and acoustic Ns. Glossopharyngeal N. 



Trochlear N. 



Trigeminal N. 



Vagus N. 



Accessory N. 

 Hypoglossal N. 



Mesencephalon L 

 Oculomotor N. _{ 



Diencephalon 



Cerebral 

 hemisphere 



Froriep's 

 ganglion 



I. Cermeal 



-H Phrenic N. 



I. Thoracic 



Vitelline loop 



Tail (i 



/. Coccygeal 4 



'/. Lumbar 



I. Sacral 



Fio. 645. Reconstruction of periphera nerves of a human embryo of 10.2 mm. (After His.) The abducent nerve 

 is not labelled, but is seen passing forward to the eye under the mandibular and maxillary nerves. 



The second bend appears at the junction of the hind-brain and medulla spinalis. 

 This is termed the cervical flexure (Fig. 652), and increases from the third to the 

 end of the fifth week, when the hind-brain forms nearly a right angle with the 

 medulla spinalis; after the fifth week erection of the head takes place and the cervi- 

 cal flexure diminishes and disappears. The third bend is named the pontine flexure 

 (Fig. 652), because it is found in the region of the future pons Varoli. It differs 

 from the other two in that (a) its convexity is forward, and (6) it does not affect 



47 





