DISSECTION OF THE BACK AND THORAX. 



HI 



ture it is comparable to a serous membrane. Like such membranes, it 

 encloses a cavity, or sac, and consists of a parietal and a visceral portion. 

 The sac is known as the arachnoid cavity, or sub-dural space, receiving 

 the latter designation from its relation to the dura mater. The parietal 

 division of the membrane is represented by a layer of endothelium lining 

 the inner surface of the dura mater, to which it gives a smooth and glisten- 

 ing aspect, but from which it is not separable by dissection. The vis- 

 ceral division invests the cord and pia mater as a thin transparent mem- 

 brane, but it does so loosely, leaving a 

 potential space between it and the outer 

 surface of the pia mater. This, which 

 is the sub-arachnoid space, contains a 

 variable amount • of an alkaline fluid 

 — the sub-arachnoid fluid, which acts 

 as a water-bed to the cord. As the 

 roots of the spinal nerves extend out- 

 wards they take with them a covering 

 from the visceral arachnoid ; and, where 

 they pierce the dura mater, this cover- 

 ing becomes continuous with the parietal 

 layer. 



The Pia Mater is the vascular mem- 

 brane of the cord. It consists of areolar 

 connective-tissue in which the vessels 

 subdivide before entering the cord. It 

 invests the cord closely, and is intimately 

 connected to it ; sending a considerable 

 process into the inferior median fissure, 

 and numerous other slender filaments 

 which penetrate the substance of the 

 oord. On each side it is connected to 

 the inner surface-of the dura mater by 

 a series of pointed processes constituting 



Fig. 17. 



View of the Membranes ok the 

 Spinal Cord {Ellis). 



a. Dura mater cut open and reflected ; 

 b. Small part of the translucent arach- 

 noid, left ; h. Pia mater closely investing 

 the spinal cord ; c. Ligamentum denticu- 

 latum on the side of the cord, shown by 

 cutting through the inferior roots of the 

 nerves ; d. One of the processes joining 



the ligamentum denticulatum. Each of ifc to the dura mater; g. Middle spinal 



artery ; e. Inferior roots of the nerves, 



these processes of pia mater passes out- cut;/. Superior roots. 

 wards from the side of the cord, and, 



carrying the arachnoid with it, becomes attached to the dura mater, 

 midway between the points of perforation of the superior and inferior 

 nerve roots. Behind the point in the sacral region at which the spinal * 

 cord stops, the pia mater is prolonged as an attenuated thread — the 

 plum terminable — which is enclosed by, and blends with, the tapering 

 end" of the dura mater. 



Directions. — Before the removal of the cord the student should observe 

 i^s varying thickness at different points, and the disposition of the spinal 

 nerves within the spinal canal. 



