3 i 4 NORMAL HISTOLOGY. 



The numerous nerves within the pia mater are chiefly sympathetic 

 fibres destined for the walls of the blood-vessels. Within the skull they are 

 derived chiefly from the plexuses surrounding the internal carotid and ver- 

 tebral arteries; within the spinal pia they are contributed usually from the 

 gray sympathetic rami. Additional nerve-fibres, probably sensory in func- 

 tion, occur in small numbers. Their mode of termination is uncertain, 

 although free and bulbus endings have been described. 



The Arachnoid. The intermediate membrane is, for the most part, 

 a thin connective tissue envelope that intervenes between the dura and the 

 pia and, notwithstanding its delicacy, completely separates the subdural 

 from the subarachnoid space. It contains neither blood-vessels, lymphatics 

 nor nerves and consists of an interlacement of flattened bundles of fine 

 fibrous tissue interspersed with elastic fibres and plate-like cells. In addi- 

 tion to the main sheet, the partition, both sides of which are covered with 

 endothelium, numerous trabeculae, also covered with endothelium, extend 

 across the subarachnoid space and in places are so plentiful as to convert 

 the cleft into a sponge-like structure. In contrast .to the pia mater, which 

 closely follows the surface of the brain and cord, the arachnoid is separated 

 from the cerebro-spinal axis and its immediate covering by a more or less 

 extensive space. Over the convexities of the convolutions, however, the 

 arachnoid and the pia are fused into a single membrane; elsewhere the sub- 

 arachnoid space, filled with cerebrospinal fluid, is considerable and on the 

 basal surface of the brain very extensive and represented by the cisterns. 



Not only by lymph-paths along the nerve-trunks and larger veins, the 

 cerebro-spinal fluid also escapes into the dural sinuses by filtration through 

 local tuft-like accumulations of arachnoid tissue, situated particularly along 

 the superior longitudinal sinus. These tufts, known as the Pacchonian 

 "bodies, consist of spongy masses of arachnoid tissue, covered externally 

 with endothelium, which push before them the greatly attenuated dura and, 

 overlaid by the latter and the endothelial lining of the blood-space, project 

 into the sinus or its lateral diverticula. By this arrangement the cerebro- 

 spinal fluid that occupies the interstices of the arachnoid tissue filters through 

 the interposed structures and finds its way into the venous current within the 

 dural sinuses. 





