DISSECTION OF THE BACK AND THORAX. 137 



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 vertebrte, 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 vertebi-se 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 Meninges, of the Spinal Cord. 



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

 is the protective envelope of the cord, and has the form of tubular mem- 

 brane of fibrovis 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 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 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 backwai-ds 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- 

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

 encloses a cavity, or sac, and consists of a 2)(iri€tal and a visceral portion. 

 The sac is known as the arachnoid cavity, or suh-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 Avhich 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 space between it and the outer 



