278 PRACTICAL ANATOMY. 



surface of the dura mater. Its fibrous structure is but 

 faintly developed, and it adheres intimately to the inner 

 surface of the dura mater. Its visceral layer adheres 

 loosely to the brain, passing from convolution to convo- 

 lution without dipping down between them. These 

 views are at variance with those now ordinarily received, 

 which deny the existence of the arachnoid as a distinct 

 serous sac. Embryologically, however, the arachnoid 

 exists as such. In young dogs its fibrous and epithelial 

 elements are strongly developed, and leave no question as 

 to its classification among the serous membranes. The 

 space between the two layers of the arachnoid is called 

 the cavity of the arachnoid. The arachnoid is thin and 

 delicate over the convexity of the brain; thicker at the 

 base, where it is separated from the pia mater forming the 

 subarachnoid space. The subarachnoid space contains 

 a varying amount of clear, watery fluid, which consists 

 of 98.5 water and 1.5 parts of solids. As the sub- 

 arachnoid space communicates with the ventricles of the 

 brain, the office of this fluid is probably to support the 

 ventricular walls and to modify the force of concussions 

 transmitted to the brain. 



THE PIA MATER. 



The pia mater is a vascular membrane, which closely 

 invests the brain. It is composed of loose connective 

 tissue, containing elastic fibres and supporting plexuses 

 of blood-vessels. It passes through the transverse fissure, 

 and forms the velum interpositum of the third ventricle 

 and the choroid plexuses of the lateral and fourth ven- 

 tricles. Its outer surface is covered by the visceral layer 

 of the arachnoid. The surface applied to the brain 

 exhibits numerous vessels that come off to pass to the 

 cortical portion of the brain, and, if immersed in water, 



