412 HISTORY OF THE HUMAN BODY 



greater or less differentiation of its anterior portion, which 

 forms the metencephalon in a restricted sense (the cerebellum 

 of higher forms), while the posterior portion, which tapers in- 

 definitely into the spinal cord, is distinguished as the myelen- 

 cephalon or medulla.* The ventricle of the third primary vesi- 

 cle, or more especially that of the myelencephalon, is a large 

 and conspicuous cavity in lower vertebrates and in the embryo 

 of the higher ones, and is known as the fourth ventricle. That 

 part of the lumen which lies between this and the third ven- 

 tricle, including the ventricle of the mid-brain, forms in Man 

 a small tube or duct, and has consequently received the name 

 of aqueductus cerebri \_Sylvii], or the " iter a tertio ad quar- 

 tum ventriculurn." 



The original three primary cerebral vesicles, by a secondary 

 subdivision of the first and third, thus become increased to five, 

 and form a fundamental plan to which the brain of all higher 

 vertebrates may be referred. In the development of the many 

 forms of adult brains from this ground plan certain mechanical 

 principles are involved which it is well to consider separately 

 before continuing the special history. These mechanical 

 principles are as follows : 



i. Increase in the thickness of the wall over a definite area. 



*The ease with which the German anatomists have translated into the 

 vernacular the somewhat ponderous Greek terms for the parts of the 

 brain (Vorderhirn, Zwischenhirn, Mittelhirn, etc.) has led to various 

 attempts on the part of English-speaking scholars to emulate their ex- 

 ample, but with varied success. Thus " fore-brain " and " mid-brain " for 

 prosencephalon and mesencephalon respectively are convenient although 

 somewhat mediaeval, and these, together with the inelegant " hind- 

 brain," are now in general use. The forms " after-brain " for myelen- 

 cephalon (Ger. Nachhirn) and " twixt-brain," or " tween-brain " for 

 diencephalon (Ger. Zwischenhirn) are less happy, and it is doubtful 

 if they will ever receive general favor. The Greek terms are, on the 

 whole, the most satisfactory, and are more in accordance with our usage 

 than are their rather crude Anglo-Saxon equivalents. 



The numbering of the cerebral ventricles is that of an old enumeration 

 and does, not at all correspond with the morphological value of the parts. 

 They are most conveniently named in accordance with the vesicles of which 

 they form the cavities, thus: telocozles, diaccele, mesoccele, metaccele and 

 myeloccele. 



