

MEMBRANES OF THE CORD. 



501 



253. The Spinal Cord and 

 its Membranes. 



mater being continued in the form of a tubular prolongation on them as they issue 

 from 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 inclose 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 dara 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 

 tissue, arranged in bands which intersect one another. 

 It is sparingly supplied with vessels ; and no nerves 

 have as yet been traced into it. 



The Arachnoid is exposed by slitting up the dura 

 mater, and reflecting this membrane on either side 

 (fig. 253). It is a thin, delicate, serous membrane, 

 which invests the outer surface of the cord, and is 

 then reflected upon the inner surface of the dura 

 mater, to which it is intimately adherent. That 

 portion which surrounds the cord is called the vis- 

 ceral layer of the arachnoid; and that which lines 

 the inner surface of the dura mater, the parietal 

 layer; 1 the interval between the two is called the 

 cavity of the arachnoid. The visceral layer forms 

 a loose sheath around the cord, so as to leave a con- 

 siderable interval between the two which is called 

 the sub-arachnoidean space. This space is largest 

 at the lower part of the spinal canal, and incloses 

 the mass of nerves which fbrm the cauda equina. It 



contains an abundant serous secretion, the cerebro-spinal fluid, and usually com- 

 municates with the general ventricular cavity of the brain, by means of an opening 

 in the fibrous layer of the inferior boundary of the fourth ventricle. This 

 secretion is sufficient in amount to ex- 

 pand the arachnoid membrane, so as to 

 completely fill up the whole of the space 

 included in the dura mater. The sub- 

 arachnoidean space is crossed, at the back 

 part of the cord, by numerous fibrous 

 bands, which stretch from the arachnoid 

 to the pia mater, especially in the cervi- 

 cal region, and is partially subdivided 

 by a longitudinal membranous partition, 

 which serves to connect the arachnoid 

 with the pia mater, opposite the posterior 

 median fissure. This partition is incom- 

 plete, and cribriform in structure, consisting of bundles of white fibrous tissue, 

 interlacing with each other. The visceral layer of the arachnoid surrounds the 

 spinal nerves where they arise from the cord, and incloses them in a tubular 



Fig. 254. Transverse Section of the Spinal 

 Cord and its Membranes. 



1 Kb'lliker denies that the inner surface of the dura mater is covered by an outer layer of the 

 arachnoid, and states, that nothing is found here except an epithelial layer, no trace of a special 

 membrane existing. 



