DUKA MATER 667 



the area praecentralis anterior (Fig. 594). Most of the motor area is hidden in the 

 sulcus centralis, but towards the supero-medial margin of the hemisphere a con- 

 siderable area emerges upon the surface of both the gyrus centralis anterior and 

 the paracentral lobule. 



Brodmann calls the rest of the frontal territory the regio frontalis ; but in the 

 colour scheme adopted in Figs. 585, 589, and 594 the inferior frontal gyrus and 

 the orbital area posterior to the orbital sulcus have been associated with the 

 " precentral " rather than the " frontal " regions. 



WEIGHT OF THE BRAIN. 



The average weight of the adult male brain may be said to be about 1360 

 grammes. The female brain weighs rather less, but this is to be expected from the 

 smaller bulk of the female body. Probably the relative weight of the brain in the 

 two sexes is very much the same. The variations met with in brain-weight are 

 very great, but it is doubtful if normal intellectual functions could be carried on in 

 a brain which weighs less than 960 grammes. In microcephalic idiots brains of 

 extremely small size are met with. 



THE MENING-ES OF THE ENCEPHALON AND 

 MEDULLA SPINALIS. 



The brain and spinal medulla are enclosed within three membranes, which are 

 termed the meninges or meningeal membranes. From without inwards these 

 are: (1) the dura mater, (2) the arachnoid, and (3) the pia mater. The space 

 between the dura mater and the arachnoid receives the name of subdural space, 

 while the much more roomy interval between the arachnoid and the pia mater is 

 called the subarachnoid space. 



DURA MATER. 



The dura mater is a dense and thick fibrous membrane which possesses a very 

 considerable degree of strength. Its arrangement within the cranial cavity is so 

 different from that within the vertebral canal that it is customary to speak of it as 

 consisting of two parts, viz., a cranial and a vertebral, although in adopting this sub- 

 division it must be clearly understood that both portions are continuous with each 

 other at the foramen magnum. 



Dura Mater Encephali. The cranial dura mater is adherent to the inner 

 surface of the cranial wall, and performs a double office. It serves as an internal 

 periosteum for the bones which it lines and it constitutes an envelope for the brain. 

 Its inner surface, which bounds the subdural space, is smooth and glistening, and 

 is covered with a layer of endothelial cells. The outer surface when separated from 

 the cranial wall, is rough, this being due to numerous fine fibrous processes and 

 blood-vessels which pass between it and the bones. Its degree of adhesion to 

 the cranial wall differs considerably in different regions. To the vault of the 

 cranium, except along the lines of the sutures, the connexion is by no means 

 strong, and in the intervals between the fibrous processes which pass into the bone 

 there are small lymph spaces (epidural spaces) where the outer surface of the 

 membrane is covered by endothelial cells. So long as the sutures are open the 

 dura mater is connected with the periosteum on the exterior of the skull, along the 

 sutural lines, by a thin layer of fibrous tissue which intervenes between the bony 

 margins. Around the foramen magnum, and on the floor of the cranium, the dura 

 mater is very firmly adherent to the bone. This is more particularly marked in the 

 case of the projecting parts of the cranial floor, as, for example, the petrous portions 

 of the temporal bones, the clinoid processes, and so on. This firm adhesion in 

 these regions is still further strengthened because the nerves, as they leave the 

 cranium through the various foramina, are followed by sheaths of the fibrous dura 



