514 



THE NEEVOUS SYSTEM. 



TELENCEPHALON 



Anterior limit of mesencephalon 



PROSENCEPHALON 



Anteri( 

 nenrophore M 



Skin 



Recessus mamillaris' 



Upper limit of 

 dp* ^rhombencephalon 



Even at a time, during the second week, when the anterior (oral) end of the 

 neural tube is still open (neuroponis anterior), a right-angled bend has already 

 developed in the rudiment of the brain (cerebral vesicle). Slightly less than half 

 of the length of the vesicle had projected beyond the upper (anterior) end of the 



no fcochord and became flexed 

 ventrally round it (Fig. 

 455). 



This bend is known as 

 the cephalic flexure. The 

 region of the brain vesicle 

 in which it develops will 

 later on become the mesen- 

 cephalon or mid-brain; and 

 even at the early stage of 

 development now under 

 consideration (Fig. 455) 

 there is a slight narrowing 

 of the tube (isthmus) that 

 marks the boundary be- 

 tween the mid-brain and 

 the rhombencephalon or 

 hind -brain. Just beyond 

 the end of the notochord 

 there is an even fainter 

 trace of a constriction in- 

 dicating the line of de- 

 marcation between the 

 mid-brain and the prosen- 

 cephalon or fore-brain. 

 Shortly after the appearance of the cephalic flexure a similar bending occurs 

 in the region where the encephalon becomes continuous with the medulla spinalis 

 (Fig. 456, A). This is the cervical flexure. 



But at this stage, or even earlier (Fig. 456), there has been developing a third 

 bend which produces effects differing from those just mentioned. At the end of 

 the second week a slight bulging can be detected on the ventral side of the hind- 



Upper limit of 

 spinal medulla 



FIG. 455. LEFT LATERAL ASPECT OF AN EARLY HUMAN EMBRYO 

 (after His's model, reversed). 



CEREBRAL 



HEMISPHERE: 



OPTIC VESICLE: 



A B 



FIG. 456. Two STAGES IN THE DEVELOPMENT OF THE HUMAN BRAIN (after His). 

 A, Brain of an embryo of the third week. B, Brain of an embryo of five weeks. 



brain (Fig. 455) : during the next four weeks this steadily becomes accentuated anc 

 forms the pontine flexure. The convexity of the bend is directed ventrally 

 differing in this respect from both of the other flexures. This difference ii 

 direction has a profound influence upon the form which the hind-brain assumes 

 If a plastic tube is bent a strain is thrown upon the wall in the concavity 



