BRAIX. 67 



It is perhaps chiefly characterised by the small development of 

 the cerebral or prosencephalic part of the fore-brain. The anterior 

 end of the medullary tube becomes at an early embryonic stage, 

 when its walls are still epithelial, differentiated by two constric- 

 tions into three vesicles, the fore, mid, and hind cerebral vesicles. 

 Of these the posterior vesicle or hind-brain gradually tapers 

 behind into the spinal cord, and the portion of the medullary 

 canal contained in it gives rise to the fourth ventricle of the adult. 

 Its walls become transformed into the medulla oblongata, which 

 is a development of the floor and sides of the hind-brain and is 

 frequently called the myelencephalon. The cerebellum (meten- 

 cephalon) is a special development of the anterior part of the 

 dorsal wall of the hind-brain. The greater part of the dorsal 

 wall remains throughout life at the epithelial stage and never 

 develops nervous tissue. The mesoblast (pia mater) overlying 

 this permanently epithelial wall becomes especially vascular 

 and gives rise to the choroid plexus of the fourth ventricle. 



The middle vesicle or mid-brain (mesencephalon) gives rise by 

 its roof and sides to the optic lobes or corpora bigemina, and by 

 its floor to a stout nervous mass consisting largely of strong 

 bundles of nerve fibres which in the mammalian brain constitute 

 the crura cerebri or peduncles of the cerebrum. The portion of the 

 medullary canal in the mid-brain is the iter a tertio ad quartum 

 ventriculum or aqueductus sylvii. 



The anterior vesicle or fore-brain becomes early differentiated 

 into three parts ; a posterior part, the thalamencephalon, the 

 central canal of which constitutes the third ventricle ; a ventral 

 part usually described as part of the thalamencephalon, the 

 infundibulum ; and an anterior part the proscncephalon or cere- 

 brum, the ventricle of which is the second ventricle. The cere- 

 brum is usually divided into a right and left lobe by a longitu- 

 dinal vertical constriction, but this happens rarely (Dipnoi, 

 Marsipobranchii) in fishes, though there are sometimes indica- 

 tions of this division in the form of a longitudinal surface 

 groove, and in Elasmobranchs the contained ventricle is ac- 

 tually divided into a right and left ventricle which open be- 

 hind into the third ventricle by the foramen of Munro and 

 are termed the lateral ventricles. 



The anterior end of the cerebrum is always marked off as two 

 lobes of varying size and shape into which the second ventricle 



