THE SPINAL CORD 



731 



THE SPINAL CORD 



The spinal cord is the elongated portion of the cerebro-spinal axis which is 

 contained within the spinal canal. It extends from the level of the transverse 

 ligament of the atlas to the ))ody of the second lumbar vertel)ra. Occasionally it 

 only extends as far as the body of the first lumbar vertebra. It is invested, in 

 common with the brain, by three membranes: dura mater, arachnoid, and pia 

 mater. The portions of these membranes which are contained within the cranial 

 cavity, and are therefore in relation to the l^rain, have been already described (page 

 671). There are, however, certain differences betAveen the cranial and the spinal 

 meninges; therefore, a short separate description of the latter will be necessary. 



Dissection. — The subject being i)laeed on its face, the neural canal may be laid open in the usual 

 manner by cutting through the pedicles of the vertebrae and removing the neural arches of the 

 saciTim. The cord and its membranes should be first examined in situ. About three inches of 

 the theca should be ripped U]) at the junction of the dorsal and lumbar regions for this purpose. 

 The entire cord with the membranes should then be removed by dividing both cord and mem- 

 branes at the level of the articulation between the atlas and axis and cutting through the nerves 



Fig. 435. — Transverse Section of the Spinal Cord and its Membranes. 

 (After Key and Retzius.) 



SUBARAfUyoiD SPACE 



A y TERIOR XER I E-R 00 TS 

 (in stctiun) 



Ligamentum denticulatum 



Dura mater - 



SlBARACJ/yOW 

 TRABECl LA WITH 

 BVyULES OF POSTERIOR 

 XERVE-ROOTS 



Septum poBticum 



as they enter the intervertebral foramina. Two or three of the lower lumbar or upper sacral neives 

 should be cut sufficiently long to preserve the ganglia on the posterior roots. The cord should 

 now be pinned out with its posterior surface uppermost in a cork-lined tray and dissected under 

 water. On laying open the dura mater by a mesial longitudinal incision, the arachnoid will come 

 into view and will be rendered more evident ]jy blowing air between it and the pia mater with a 

 blow])ipe. On slitting up the arachnoid some of the coarser strands of the subarachnoid tissue 

 may be made out. The posterior and anterior nerve-roots will also be seen with the ligamentum 

 denticulatum between them. The connection of the latter with the pia mater is well seen under 

 water. The pia mater appears white and glistening, and when traced downwards is verj- 

 obviously C(jntinuous with the filum terminale. The last-named stiTicture stands out conspicuously 

 am(jng the duller-coloured nerve-roots which constitute the eauda equina. On the anterior asjject 

 of the cord the linea splendens will be seen. Lastly the student should make a series of trans- 

 verse sections through the cord and compare the disposition of the grey and white matter in the 

 different regions (fig. 439). 



The DURA MATER forms a loose sheath or theca around the cord. It invests 

 not only the cord but also the elongated nerve-roots, which (under the name of cauda 

 equina) extend beyond the cord. It invests the cauda equina as far as the second 

 or third sacral vertebra, but at this level it converges so as to form a blunt hollow 

 cone, and is represented lower down only by the sheath of the filum terminale. By 

 means of this sheath it is attached to the base of the coccyx. Its outer surface is 



