DISSECTION OF THE BACK AND THORAX. 



141 



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 svh-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 divisiou 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-archnoid 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 

 cord. On each side it is connected to 

 the inner surface of the dura mater by 

 a series of pointed processes constituting 

 the ligamentum denticulatum. Each of 



Fig. 17. 



artery; e. Inferior roots of the nerves, 

 cut ; f. Superior roots. 



View of the Membranes of the 

 Spinal Cord {Ellis). 



I. Dura mater cut open and reflected ; 

 „ . , „, ■ o. Small part of the translucent arach- 



these processes ot pia mater passes out- ^^j^^ i^j^ . /j pjg^ nj^ter closely investing 

 wards from the side of the cord, and, ]!:Z:nZtA: ^^:^Tl^^y 

 carrying the arachnoid with it, becomes ^:i^l:"S^ :^^^::,:::^:L^ 

 attached to the dura mater, midway be- it to the dura mater; g. Middle spmai 

 tween the points of perforation of the 

 superior and inferior nerve-roots. Be- 

 hind the point in the sacral region at which the spinal cord stops, the pia 

 mater is prolonged as an attenuated thread— the filuin terminate— vfhich 

 is enclosed by, and blends with, the tapering end of the dura mater. 



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

 its varying thickness at diff'erent points, and the disposition of the spinal 

 nerves within the spinal canal. 



