THE BEAIN. 



511 



wards on the body, is commencing to appear at the junction of 

 brain and spinal cord ; it is shown in Fig. 215, at a level between 

 the reference lines lie. 3 and en. 



The fourth week. By the end of the fourth week the shape 

 of the brain is as shown in Fig. 21G. The flexure at the level 

 of the mid-brain, or mesencephalic flexure as it may be termed, 

 has increased greatly in extent, and now amounts to about 180°, 



HC.1 



HC.3 



Fig. 215. — The head and fore part of the body of a Human Embryo lettered 

 by Professor His, Lr, and estimated as twenty or twenty-one days old. 

 (Cf. Fig. 198.) The brain is exposed from the left side ; the rest of the 

 embryo is represented in sagittal section. (From His.) x 28. 



BP, thalamencephalon. BH, hind-brain, or medulla oblongata. BL, cerebellum. 

 BM, mid-brain. BO, optic vesicle. BS, vesicle of the cerebral hemispheres. CH, 

 notochord. EI, auditory vesicle. HC.l, first branchial pouch. HC.3, third bran- 

 chial pouch. HM, hyomand ibul ar pouch. MIST, mandibular arch. BT, truncus 

 arteriosus. TO, oesophagus. "W, liver. 



the infundibulum and the hind-brain almost touching each other. 

 The cervical flexure, marking the junction of the brain and spinal 

 cord, at the level of the reference line a. 5, is also much more 

 pronounced than before, and forms an angle of about 90°. A 

 third, or metencephalic flexure, with the concavity directed 

 dorsal wards, is commencing to form opposite the cerebellum , 

 at the level of the reference line pt ; at a slightly later stage 

 this flexure becomes very strongly marked. 



As regards the individual parts of the brain, the vesicle of 



