428 THE ORGANS 



segmental neurones) and still others are suprasegmental associative 

 neurones, confined to suprasegmental structures (Fig. 296). 



MEMBRANES OF THE BRAIN AND CORD 



The brain and cord are enclosed by two connective-tissue mem- 

 branes, the dura mater and the pia mater, a part of the latter being 

 often referred to as a separate membrane, the arachnoid (Fig. 297). 



The dura mater is the outer of the two membranes and consists 

 of dense fibrous tissue. The cerebral dura serves both as an investing 

 membrane for the brain and as periosteum for the inner surfaces 

 of the cranial bones. It consists of two layers: (a) An inner layer 

 of closely packed fibro-elastic tissue containing many connective- 

 tissue cells, and lined on its brain surface with a single layer of fiat 

 cells; and {h) an outer layer, which forms the periosteum and is similar 

 in structure to the inner layer, but much richer in blood-vessels and 

 nerves. Between the two layers are large venous sinuses. The 

 spinal dura corresponds to the inner layer of the cerebral dura, which 

 it resembles in structure, the vertebras having their own separate peri- 

 osteum. The outer surface of the spinal dura is covered with a single 

 layer of fiat cells, and is separated from the periosteum by the 

 epidural space, which contains anastomosing venous channels lying 

 in an areolar tissue rich in fat. The spinal dura is said to contain 

 lymphatics which open on both of its surfaces. Beneath the spinal 

 dura, between it and the arachnoid, is the subdural space, a narrow 

 cleft containing a fluid probably of the nature of lymph. It is stated 

 that this space communicates by lymph clefts with the lymph spaces 

 ■ in the sheaths of nerves and also communicates with the deep lymph- 

 atic vessels and glands of the neck and groin. It has no direct com- 

 munication with the subarachnoid space (see below). 



The pia mater closely invests the brain and cord, extending into 

 the sulci and sending prolongations into the ventricles. It consists 

 of fibro-elastic tissue arranged in irregular lamellae, forming a spongy 

 tissue, the cavities of which contain more or less fluid. The outer 

 lamellae are the most compact, and are covered, on the dural surface 

 by a single layer of fiat cells. It is this external layer of the pia which 

 is frequently described as a separate membrane, the arachnoid. The 

 space beneath the arachnoid is the subarachnoid space and contains 

 cerebrospinal fluid. There is direct communication in the roof of the 

 fourth ventricle between the subarachnoid space and the brain cavity. 



