THE NERVOUS SYSTEM 1481 



It can enter the lymph-spaces of the cranial nerves upon which 

 the arachnoid is prolonged outwards in the form of sheaths; and 

 it can enter the lacunae laterales through the intervention of the 

 Pacchionian bodies, and from these lacunae it can make its way 

 into the superior longitudinal sinus. 



Structure of the Arachnoid Membrane. — The arachnoid consists of fine 

 fibrous tissue, arranged in interlacing bundles, the intervals between these 

 bundles being occupied by delicate cellular membranes. Several such layers, 

 intimately blended together, form the membrane. 



Beneath the arachnoid, and constituting a part of it, there is a reticulum 

 of subarachnoid trabecules. These trabeculae consist, as in the case of the 

 arachnoid proper, of fine fibrous tissue, but the intertrabecular spaces, instead 

 of being occupied by cellular membranes, contain cerebro-spinal (subarach- 

 noid) fluid. The trabecular reticulum connects the arachnoid \\-ith the 

 subjacent pia mater. The superficial surface of the arachnoid is covered 

 ■w-ith a delicate layer of endothelium. 



The nerves of the arachnoid are derived from the sympathetic 



system. 



Pacchionian Bodies. — These are small fleshy-looking bodies which 

 are situated along the course of the superior longitudinal sinus, into 

 which some of them project. They are also met with in connection 

 with other sinuses, e.g., the lateral and straight sinuses. Each 

 Pacchionian body is a localized projection of the arachnoid mem- 

 brane, with which it is connected by a narrow pedicle. As stated, 

 some Pacchionian bodies project into the superior longitudinal 

 sinus ; others project from below into the lacunae laterales. In all 

 cases the bodies carry before them the lining of the sinus. Super- 

 ficially the bodies give rise to the Pacchionian depressions on the 

 internal surface of the parietal bone near the superior border. 

 Each Pacchionian body, as stated, is a localized projection of the 

 arachnoid membrane (enlarged villus — Luschka). It contains a 

 prolongation of the subarachnoid space and reticulum. This is 

 surrounded by a prolongation of the arachnoid membrane, and 

 external to this is the endothelial lining of the sinus or of the 

 lacuna. The Pacchionian bodies constitute channels through which 

 the subarachnoid fluid can be drained away from the subarachnoid 

 space into the lacunae laterales, and thence into the superior longi- 

 tudinal sinus, as well as into the other sinuses, e.g., the lateral 

 and straight sinuses. They are quite rudimentary in early life, and 

 are most conspicuous in advanced life. 



Pia Mater. 



The pia mater is the most internal covering of the encephalon. 

 It is a very vascular membrane, which invests, and is closely 

 adherent to, the entire surface. From its internal surface delicate 

 processes pass into the cerebral substance, which represent the 

 minute bloodvessels, surrounded by pia-matral sheaths. The pia 

 mater not only invests the external surface, but also dips into the 

 sulci, and covers the opposed surfaces of the gyri. It also furnishes 



