GENERAL DEVELOPMENT OF THE BRAIN. 



1059 



of unequal growth in two slight constrictions separating three dilatations known as 

 the primary brain -vesicles. The posterior of these, the hind-brain,' is much 

 the longer, exceeding the combined length of the other two (Fig. 911); altera short 

 time when viewed from behind it presents an elongated lozenge-shaped form and, 

 hence, is also called the rhombencephalon. The middle vesicle, the mid-brain, 

 or mesencephalon, is conspicuous on account of its rounded form and prominent 

 position, lying, as it does, over the marked primary flexure which the head-end of 

 the neural tube very early exhibits. 



The anterior vesicle, known as the fore-brain, or prosencephalon, at first is 

 small and rounded, but soon becomes modified by the appearance, on either side, 

 of a hollow protuberance, the optic vesicle, that pushes out from the lower lateral 

 wall. For a time the optic vesicle communicates with the main cavity of the fore- 

 brain by a wide opening. This gradually becomes reduced and constricted until the 



A 



Fig. 911. 



Fore-brain 



Pallium 



Mid-brain 



Optic vesicle 



Fore-brain 

 (thalamic region) 



Pallium 



Mid-brain 



Hind-brain 



Reconstruction of brain of human embryo of about two weeks (3.2 mm.); A, outer surface; B, inner surface; 

 up, neural pore, where fore-brain is still open; cj, anlage of corpus striatum; or, optic recess leading into optic 

 vesicle; A/, hypothalamic region. [His.) 



evagination is attached by a hollow stem, the optic stalk, which later takes part in 

 the formation of the optic nerve that connects the eye with the brain, the vesicle 

 itself giving rise (page 1482) to the nervous coat of the eye, the retina. By the 

 time the optic evagination is formed, the front part of the fore-brain shows a slight 

 bulging, narrow below and broader and rounded above, and separated from the 

 optic outgrowth by a slight furrow. This is the first suggestion of the anlage of the 

 hemisphere or pallium (His). The latter soon gives rise to two rounded hollow 

 protrusions, one on either side of the fore-brain, that rapidly expand into the 

 conspicuous primary cerebral hemispheres. The lower part of the fore-brain includes 

 the region that later, after differentiation and outgrowth from the hemisphere, 

 receives the nerves of smell and is known as the rhinejicephalon. 



A slight ridge (Fig. 911, ^), projecting inward from the roof of the fore-brain, 

 suggests a subdivision of the general space into a posterior and an anterior region. 



^This use of the term hind-brain is at variance with its older significance, still retained by 

 some German writers, as indicating the upper division (metencephalon) of the posterior 

 primary vesicle. In view, however, of the now general application of fore-brain and mid-brain 

 to the other primary vesicles, it seems more consistent to include hind-brain in the series, as has 

 been done by Cunningham, with a distinct gain not only in convenience, but in avoiding terms 

 which in their Anglicised form are at best awkward and unnecessary. 



