MEMBRANES OF THE CORD. :>39 



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



and spines of the vertebrae, the spinal canal is to be opened the 8 cord 

 by sawing through the laminae 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 neural arches. 



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

 and by a loose areolar tissue containing fluid sometimes, especially m 

 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 invertebral foramina are to be denned. 



MEMBRANES OF THE CORD (figs. 190 and 195, p. 540). Three Spinal 

 membranes, like those on the brain, surround the cord, viz., an are three 

 external tube of dura mater, an internal covering of pia mater, *" number. 

 and an intermediate sheath of arachnoid. 



The DURA MATER (a) is the strongest tube, and is continuous Dura mater 

 with the membrane lining the interior of the skull. It forms a surrounds 

 loose sheath (theca) along the spinal canal as far as the last lumbar cord loosel y 

 vertebra ; and then tapering gradually it ends opposite the second lower 

 or third piece of the sacrum in a slender impervious cord which is en 

 continued to the back of the coccyx (fig. 194 6). The capacity of 

 the sheath greatly exceeds the dimensions of the cord ; and it is size of 

 larger in the neck and loins than in the dorsal region. 



On the outer aspect the spinal dura mater is smooth, in com- connec- 

 parison with that in the skull, for it does not act as a periosteum tlons 

 to the bones. Between it and the wall of the canal are some vessels 

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

 the vertebrae by a few fibrous bands. 



On each side the durer mata sends offsets along the spinal nerves offsets on 

 in the iutervertebral foramina ; and these offsets become gradually nervcs ' 

 longer below (fig. 194), where they form tubes which enclose the 

 sacral nerves, and lie for some distance with the spinal canal. In median 

 the centre between the lowest offsets on the nerves, is the slender 

 fibrous cord (6), which blends with the periosteum covering the 

 back of the coccyx. 



Dissection. To remove the spinal cord with the sheath of the Dissection 

 dura mater from the body, the lateral processes in the intervertebral cord* 

 foramina, with the contained nerves, are to be cut ; and one or two 

 of them in the dorsal region should be followed outwards beyond 

 the apertures by cutting away the surrounding bone. The central 

 prolongation may be now detached from the coccyx ; and the 

 membranes are to be divided opposite the fourth cervical vertebra, 

 and to be removed with the contained cord, which has already been 

 severed in the removal of the brain, by cutting the bands that 

 attach the dura mater to the posterior common ligament. 



When the cord is taken out, place the anterior surface uppermost, and see next 

 with the lateral offsets widely separated. To show the arachnoid c 

 covering, the dura mater is to be slit along the middle as far as the 



