THE NERVOUS SYSTEM. 



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 laminae 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. Its internal surface is covered 

 by 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. 

 400). 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 nerves, 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, connected 

 together by isolated connective-tissue trabeculre, especi- 

 ally 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 continuous with the cranial subarachnoid space, and 

 communicates with the general ventricular cavity of the brain by means of an 

 opening in the pia mater at the inferior boundary of the fourth ventricle (for- 

 amen of Majendie). It contains an abundant serous secretion, the cerebro-spinal 



fluid. This secretion is sufficient in 

 amount to expand the arachnoid mem- 

 brane, so as to completely fill up the 

 whole of the space included in the dura 

 mater. The subarachnoidean space is 

 occupied by trabeculae of delicate con- 

 nective tissue, connecting the pia mater 

 on the one hand with the arachnoid mem- 

 brane on the other. 

 arachnoid tissue. In 

 partially subdivided 



membranous partition, which serves to 

 connect the arachnoid with the pia mater, 

 opposite the posterior median fissure. This 

 partition is incomplete and cribriform in 

 structure, consisting 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 

 lymph carried to it by the perivascular lymph-sheath (see page 87) is conveyed 

 back into the circulation. 



FIG. 400. The spinal cord and 

 its membranes. 



Dura, mater 



Arachnoid 

 fost. root 



Ant. root 



This is named sub- 

 addition to this it is 

 by a longitudinal 



Dura mater 



Plexus venosus 



a vertebralia 



FIG. 401. Transverse section of the spinal cord 

 and its membranes. (Gegenbaur.) 



