138 



THE ANATOMY OF THE HORSE. 



surface of the pia mater. This, which is the sub-arachnoid sjmce, con- 

 tains a variable amoxiut 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 outwards, they take with them 

 a covering from the visceral arachnoid ; 

 and Avhere they pierce the dura mater, 

 this covering 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 denticulatuni. Each of 

 these processes of pia mater passes out- 

 FiG. 15. wards from the side of the cord, and, 



View ok the Membranes ok the Spinal caiTying the arachnoid with it, bcCOmCS 



_ " ^ ' ,1 a * J attached to the dura mater, midway be- 



a. Dura mater cut open and reflected ; ' "^ 



6. Small part of the translucent arach- twcen the poiuts of perforation of the 

 noid, left ; li. Pia mater closely investing '■ 



the spinal cord ; c. Ligamentum denticu- supsrior and the inferior iicrve-roots. 



latum on the side of the cord, sliown by-p,,.,^, • , • ^ ^ 



cutting tiirough the inferior roots of the xJehmd the point in the sacral rcgioii at 

 rt™he''dnrr nLL'! ^.'^"Sie"' spiiTal "which the spiual cord stops, the pia 

 cS=SupSrootI°"*'°'*^'""'^''' mater is prolonged as an attenuated 

 thread — the Jiium tenninale — 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 

 its varying thickness at different points, and the disposition of the spinal 

 nerves within the spinal canal. 



The spinal cord begins at the foramen magnum by continuity with 

 the medulla oblongata, and it is here of considerable thickness. Trac- 

 ing it backwards, it is seen to become thicker behind the middle of 

 the cervical region, forming the cervical enlargement, which extends as 

 far as the 2nd dorsal vertebra. It is from this enlargement that the 

 nerves which supply the fore limb are given off; Beyond the 2nd 

 dorsal vertebra the cord contracts slightly, so as to become about the 

 middle of the back smaller even than in its initial portion. Preserving 

 this diminished thickness throughout the dorsal region, it again expands 



