THE BRAIN OR ENCEPHALON 



853 



Dimensions. The sagittal or occipitofrontal diameter of the white male adult 

 brain averages l(i to 17 cm. (6.4 to 6.8- inches), the maximum width in the 

 parietal region averages 13 to 14 cm. (5.2 to 5.6 inches), while the maximum height 

 is about 12.5 cm. (5 inches). The dimensions of the female brain are usually 

 somewhat less. The brains of dolichocephalic individuals are naturally longer 

 and narrower than those of brachycephalic, and other differences in size and 

 shape are found in conformity with the cranial configuration and other factors. 



Fi<;. 619. Basal aspect of the brain showing the superficial origin of the cranial nerves. The Roman 



numerals indicate the nerves. 



The Development of the Brain and the Usual Classifications of its Sub- 

 divisions. The cephalic region of the embryonic neural plate is characterized, 

 as already pointed out (p. 808), by a rapid process of expansion and intensity of 

 growth energy which seems to indicate the higher functional potentiality of what 

 is to become the brain. The fusion of the margins of the neural plate, proceeding 

 rapidly cephalad and caudad from about the cervical region, soon effects the 

 complete closing in of the brain portion of the neural tube and its complete separa- 

 tion from the overlying ectoderm. For a brief period prior to the completion of 

 the tube formation there exists a minute opening affording communication be- 

 tween the interior of the neural tube and the surrounding amniotic cavity; this 

 temporary passage is called the neuropore (Fig. 620), and is morphologically 

 the cephalic end of the tube. Its adult position is probably in the hypophyseal 

 region. 



