596 THE ANTHROPOLOGY OF THE BRAIN. 



forward, backward, aud downward, aud more or less completely liides 

 iroiii view the remaining- and, morphologically considered, more funda- 

 mental segments. This great increase in size of the prosencephalon is 

 due to the tremendous growth of its middle layer — the nerve-tissue 

 layer. 



The cavity of the prosencephalon is called the i)rosencephalic cavity 

 (lateral ventricles of adult human anatomy); that of the thalamen- 

 cephalon, the thalamencephalic cavity (third ventricle of human anat- 

 omy), the point of communication between the two cavities being- known 

 as the foramen of Monroe. 



The cavity of the mesencephalon is called mesencephalic cavity (syl 

 vian passage, or iter a tcrtio ad qtiartum ventriculum of human anat- 

 omy). 



The cavity of the epencephalou is known as epencephalic cavity 

 (pre-axial half of the fourth ventricle) ; that of the meteucephalon is the 

 metencephalic cavity (pust-axial half of the fourth ventricle). 



The "fourth ventricle'' thus becomes a cavity common to two seg- 

 ments of the brain. 



Along the "Hoor'' of the i)rosencephalic cavity (lateral ventricle), in 

 the form of a double horse-shoe, is a portion of the brain wall in which 

 the middle (nerve-tissue) layer is wanting. At this place the epen- 

 dyma aiul ])ia mater come in contact, forming the prosencephalic velum 

 (erroneously called, inhuman anatomy, "transverse lissure"), by which 

 the integrity of the encephalic cavity, at thisi)lace, is preserved. 



Likewise there is a thalamencephalic velum (velum interpositum of 

 human anatomy) in the "roof" of the thalanieucephalon : a metencei)h- 

 alic velum in the "roof" (dorsal surface) of the nietencephalou. In the 

 latter velum is found tbe foramen of Magendie, the opening by which 

 the cerebro-spinal fluid in thC' encephalic cavities communicates with 

 that in spaces on the outside- of the pia mater known as sub-arach 

 noidian spaces. 



The nerve tissue-of the prosencephalon (cerebrum) consists of "inner" 

 or "white matter" and "outer" or "gray matter," the individual ele- 

 ments of which are bound together by a tissue called neuroglia. The 

 gray matter, otherwise called cortex, is about 3 mdlimeters in thick- 

 ness. Thecerebrum has numerous fissures aud convolutions on its 

 surface — these for the purpose of increasing the area of gray matter 

 without unduly augmenting the bulk of the brain. The sui^erticial area 

 of the cortex is about 200,000 square millimeters. 



Two of the most important fissures of the brain are the fissure of 

 Rolando and the fissure of Sylvius. In relation with the former fis- 

 sure are found the great "motor areas" of the brain, and in relation 

 with the "forking" of the latter is " Broca's center "' (the center for 

 speech). 



In the adult the cerebrum is the largest portion of the brain. The 

 I oxt largest portion of this viscus is the^cerebellum, which constitutes 

 the great bulk of the epencephalou. • 



