t 



140 THE ANATOMY OF THE HORSE. 



tion of the parts surrounding the vertebral column is apportioned 

 between the dissectors of the neck, thorax, abdomen, and pelvis, it is 

 quite impossible, without unduly interfering with what is otherwise the 

 most convenient course of dissection, to expose at once the entire cord. 

 This, however, is not a matter of much importance, since, in all the 

 main features of its structure, the spinal cord of the horse is identical 

 with that of any other mammal. The student is therefore advised to 

 study the cord of a dog or a cat, which may be exposed without 

 difficult}-. One of these animals having been secured (and perferably 

 a dog), it should be fastened to a table in the prone position, and a 

 mesial incision through the skin and muscles, down to "the vertebrae, 

 should be made from the occiput to the root of the tail. With the 

 knife the muscles are to be reflected so as to expose the vertebra? 

 as far as the junction of the arch with the body. The spinal canal 

 is then to be opened by removing the arches with a chisel and 

 mallet, or with bone-forceps. In the dorsal region each arch must 

 be disarticulated from its connection with the ribs. The spinal cord 

 enclosed within its membranes will now be exposed, and between 

 the outer membrane and the bones are some veins and a quantity 

 of fat. 



Membranes, or Meninge.% of the Spinal Cord: — 



The Dura Mater. This is the most external of the membranes. It 

 is the protective envelope of the cord, and has the form of a tubular 

 membrane of fibrous connective tissue, extending from the foramen 

 magnum, w^here it is continuous with the corresponding envelope of 

 the brain, to the posterior end of the spinal canal in the coccygeal 

 region. It does not form a tight-fitting covering to the cord, but 

 invests it somewhat loosely. Its outer surface, it is to be observed, 

 is smooth, and does not line the vertebrae, which have the ordinary 

 periosteal covering. It is connected by some slender fibrous processes 

 to the superior common ligament, The capacity of the tube varies 

 with the thickness of the cord, being greater at the atlas, lower part 

 of the neck, and lumbar region than at the intermediate points. 

 The spinal cord does not extend beyond the middle of the sacrum, 

 but the dura mater is prolonged a few inches beyond that as an im- 

 pervious, tapering process. On each side the dura mater is perforated 

 by the roots of the spinal nerves, and along these it sends offsets as far 

 as the intervertebral foramina. 



• Directions. — A small piece of the dura mater should be pinched up 

 with forceps and snipped through. Beginning at the slit thus formed, 

 it should be laid open backwards and forwards along the middle line. 

 As this is being done the membrane should be pinched up, so as to 

 prevent injury to the cord. 



The Ara\jhxoid is the second of the membranes of tne cord. It is 

 much more delicate than the dura mater, and in disposition and struc- 



