374 DISSECTION OF THE SPINAL CORD. 



CHAPTER VI. 



THE SPINAL CORD AND ITS MEMBRANES. 



THE spinal cord (medulla spinalis) gives origin to the spinal nerves, 

 and is lodged in the osseous canal formed by the bodies and neural arches 

 of the vertebrae. It is invested by prolongations of the membranes of the 

 brain, which form sheaths around and support it. 



Dissection. After all the muscles have been taken from the arches and 

 spines of the vertebrae, the spinal canal is to be opened by sawing through 

 the neural arches, on each side, close to the articular processes ; and the 

 cuts of the saw should extend to the lower end of the sacrum, but not 

 higher in the neck than the fourth cervical vertebra. As it is difficult to 

 use the saw in the hollow of the lumbar region, a chisel and a mallet will 

 be found useful to complete the division of the vertebral arches. 



The tube of the dura mater is covered by some veins and fat ; and by a 

 loose areolar tissue containing fluid sometimes, especially at the lower part. 

 The fat may be scraped away with the handle of the scalpel ; and the 

 lateral prolongations of the membrane through the intervertebral foramina 

 are to be defined. 



MEMBRANES OF THE CORD (fig. 122). Three membranes, like those on 

 the brain, surround the cord, viz., an external tube of dura mater, an 

 internal sheath of pia mater, and an intervening arachnoid or serous 

 covering. 



The dura mater (a) forms a strong tube, and is continuous with the 

 membrane lining the interior of the skull. It forms a loose sheath along 

 the spinal canal, as far as the second or third piece of the sacrum (fig. 121) ; 

 but beyond that point it is continued by a slender impervious cord to the 

 back of the coccyx (fig. 121, b). The capacity of the sheath is greater 

 than is needed for the contents ; and its size is larger in the neck and 

 loins than in the back. 



On the outer aspect the dura mater is smooth, when a comparison is 

 made between it and the part in the skull, for it does not act as a perios- 

 teum to the bones. Between it and the osseous surfaces are some vessels 

 and fat ; and it is connected to the posterior common ligament of the ver- 

 tebras by a few fibrous bands. 



On each side the dura rnater sends offsets along the spinal nerves in the 

 intervertebral foramina; and these several offsets become gradually longer 

 inferiorly (fig. 121), where they form tubes which enclose the sacral nerves, 

 and lie for some distance within the spinal canal. In the midst of the 

 tubes, below, is the slender impervious cord (), which blends with the 

 periosteum covering the back of the coccyx. 



Dissection. To remove the spinal cord witli the sheath of the dura 

 mater from the body, the lateral processes in the intervertebral apertures 

 are to be cut ; and one or two of them in the dorsal region should be fol- 

 lowed outwards beyond the intervertebral foramen by cutting away the 

 surrounding bone. The central prolongation may be now detached from 



