MEMBRANES OF THE CORD. 



573 



Fig. 320. The Spinal Cord 

 and its Membranes. 



bands which intersect one another. It is sparingly supplied with vessels ; and 

 no nerves have as yet been traced to it. 



The Arachnoid is exposed by slitting up the dura mater, and reflecting that 

 membrane to either side (Fig. 320). 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. Above, 

 it is continuous with the cerebral arachnoid; below, 

 it is reflected on the various nerves, so that its parie- 

 tal and visceral layers become- continuous with each 

 other. The visceral layer is the portion which sur- 

 rounds the cord, and that which lines the inner sur- 

 face of the dura mater is called 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 considerable 

 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 form the cauda equina. It contains an 

 abundant serous secretion, the cerebro-spinal fluid, 

 and usually communicates with the general ventricu- 

 lar 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 expand 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 cervical 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 incomplete, and cribriform in structure, 

 consisting of bundles of white fibrous tis- 

 sue, interlacing with each other. The vis- 

 ceral layer of the arachnoid surrounds the 

 spinal nerves where they arise from the 

 cord, and incloses them in a tubular sheath 

 as far as their point of exit from the dura 

 mater, where it becomes continuous with 

 the parietal layer. 



The arachnoid is destitute of vessels. No nerves have as yet been traced 

 into this membrane. 



The Pia Mater of the cord is exposed on the removal of the arachnoid 

 (Fig. 320). It is less vascular in structure than the pia mater of the brain, with 

 which it is continuous, being thicker, more dense in structure, and composed 

 of fibrous tissue, arranged in longitudinal bundles. It covers the entire surface 

 of the cord to which it is very intimately adherent, forming its neurilemma, 

 and sends a process downwards into its anterior fissure, and another, extremely 

 delicate, into the posterior fissure. It also forms a sheath for each of the fila- 

 ments of the spinal nerves, and invests the nerves themselves. A longitudinal 



1 Kolliker 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, 110 trace of a special 

 membrane existing. 



Fig. 321. Transverse Section of the 

 Spinal Cord and its Membranes. 



