438 



THE CENTRAL NERVOUS SYSTEM. 



Gray 

 matter. 



tral. The dorsal space is further divided by the septum posticum, 

 best developed in the cervical region. 



At certain points, usually along the superior longitudinal sinus, 

 the outer surface of the arachnoid is raised into villi, which are 

 covered by the inner layer of the dura, and form with the latter the 

 Pacchionian bodies or granulations. These villi are connected 

 with the arachnoid by pedicles so delicate that they often seem to 

 be suspended free in the venous current of the sinus. 



The subarachnoid space contains numerous blood-vessels, some 

 of which are free and others attached to the arachnoid. Their 

 adventitta is covered by endothelium ; hence the subarachnoid space 



would seem to assume 

 here the character of a 

 perivascular space. 



The trabeculse and 

 membranes composing 

 the arachnoid tissue show 

 a great similarity to those 

 of the mesentery, and es- 

 pecially to those of the 

 omentum. The whole 

 constitutes a typical are- 

 olar connective tissue, 

 interrupted at numerous 

 points and covered by a 

 continuous layer of en- 

 dothelial cells. Large 

 numbers of spiral fibers 

 are found here twining 

 around single or groups 

 of connective-tissue fi- 

 bers. The arachnoid 

 possesses neither blood- 

 vessels nor nerves. 



The pia mater cov- 

 ers the entire surface of 

 the brain and spinal cord, 

 dipping down into every fissure and crevice. In the spinal cord it 

 consists of an outer and an inner lamella, the former being com- 

 posed of bundles of connective tissue containing elastic fibers. 

 As a rule, the course of the fibers is longitudinal. Externally 

 this layer is covered by a layer of endothelium. The blood- 

 vessels lie between the outer and inner layers of the pia. The 

 inner layer (pia intima) is made up of much finer elements, and 

 is covered on both sides by endothelium. It is this layer which 

 accompanies the blood-vessels penetrating into the spinal cord, 

 surrounding their adventitia and forming with the latter the limits 

 of their perivascular spaces. These are in communication with the 



White 

 matter. 



Fig. 351. Section through the cerebral cortex of a 

 rabbit. The blood-vessels are injected ; X 4- 



