140 THE ANATOMY OF THE HORSE. 



tiou of the parts suiToundiiig the vertebral column is apportioned 

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

 quite impossible, without duly 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 diffi- 

 culty. One of these animals having been secured (and preferably a dog), 

 it should be fastened to a table in the prone position, and a mesial inci- 

 sion through the skin and muscles, down to the vertebrse, 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 vertebrse 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. 



21embranes, or Meninges, 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 mem- 

 brane of fibrous connective-tissue, extending from the foramen magnum, 

 where 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 vertebrse, 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 impervious, 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 Arachnoid is the second of the membranes of the cord. It is 

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



