708 



THE NERVOUS SYSTEM. 



impervious, and, ensheathing the filuin terminate, descends to the back of the 

 coccyx, where it blends with the periosteum. The dura mater is much larger than 

 is necessary for its contents, and its size is greater in the cervical and lumbar 

 regions than in the dorsal. Its inner surface is smooth. On each side may be 

 seen the double openings which transmit the two roots of the corresponding spinal 

 nerve, the fibrous layer of the dura mater being continued in the form of a tubular 

 prolongation on them as they pass through these apertures. These prolongations 

 of the dura mater are short in the upper part of the spine, but become gradually 

 longer below, forming a number of tubes of fibrous membrane, which enclose the 

 sacral nerves, and are contained in the spinal canal. 



The chief peculiarities of the dura 'mater of the cord, as compared with that 

 investing the brain, are the following : 



The dura mater of the cord is not adherent to the bones of the spinal canal, 

 which have an independent periosteum. 



It does not send partitions into the fissures of the cord, as in the brain. 

 Its fibrous lamin% do not separate to form venous sinuses, as in the brain. 

 Structure. The dura mater consists of white fibrous and elastic tissue arranged 

 in bands or lamellae, which, for the most part, are par- 

 allel with one another and have a longitudinal arrange- 

 ment. Its internal surface is covered hy a layer of 

 endothelial cells which gives this surface its smooth 

 appearance. It is sparingly supplied with vessels, and 

 some few nerves have been traced into it. 



The Arachnoid is exposed by slitting up the dura 

 mater and reflecting that membrane to either side (Fig. 

 377). It is a thin, delicate, tubular membrane which 

 invests the surface of the cord, and is connected to the 

 pia mater by slender filaments of connective tissue. 

 Above, it is continuous with the cerebral arachnoid, 

 on each side it is continued on the various neives, so 

 as to form a sheath for them as they pass outward to 

 the intervertebral foramina. The outer surface of the 

 arachnoid is in contact with the inner surface of the 

 dura mater, and the two are, here and there, joined 

 together by isolated connective-tissue trabeculse, espe- 

 cially on the posterior surface of the cord. For the 

 most part, however, the membranes are not connected 

 together, and the interval between them is named the 

 subdural space. The inner surface of the arachnoid is 

 separated from the pia mater by a considerable interval, 

 which is called the subarachnoidean space. The space 

 is the largest at the lower part of the spinal canal, and 

 encloses the mass of nerves which form the cauda equina. Superiorly it is con- 

 tinuous with the cranial subarachnoid space, and communicates with the general 

 ventricular cavity of the brain by means of an opening in the pia mater, in the 

 roof of the fourth ventricle (foramen of Majendie and foramina of Key and 

 Retzim}. It contains an abundant serous secretion, the cerebro-spinal fluid. This 

 secretion is sufficient in amount to expand the arachnoid membrane, so as to fill 

 up completely the whole of the space included in the dura mater. The subarach- 

 noidean space is occupied by trabeculae of delicate connective tissue, connecting 

 the pia mater on the one hand with the arachnoid membrane on the other. This 

 is named subarachnoid tissue. In addition to this it is partially subdivided by 

 a longitudinal membranous partition, the septum posticum, which serves to con- 

 nect the arachnoid with the pia mater, opposite the posterior median fissure of the 

 spinal cord, a partition which is incomplete and cribriform in structure, con- 

 sisting of bundles of white fibrous tissue interlacing with each other. This 

 space is to be regarded as, in reality, a great lymph space, from which the 



Dur 



FIG. 377. The spinal cord and 

 ita membranes. 



