THECA. 3 



The cord is enclosed in the vertebral canal within a sheath (theca) considerably 

 longer and larger than itself, formed by the dura mater, and separated from the walls 

 of the canal by venous plexuses, and much loose areolar tissue (fig. 2). The cavity of 

 the sheath between the pia mater and the dura mater is occupied by cerebro-spinal 

 fluid, and is divided by the curtain-like arachnoid into the spaces, subdural and sub- 

 arachnoid, above mentioned. Within the latter the cord, covered closely by pia 

 mater, is suspended, being kept in position by a ligament on each side (ligamentum 

 denticulatum), which fixes it at frequent intervals to its sheath, and by the roots of 

 the spinal nerves which pass across the space from the surface of the cord towards 

 the intervertebral foramina. 



The spinal nerves come off in pairs at intervals along the cord. The portion 

 of spinal cord to which each pair of roots is attached is termed a " segment," but 

 there is in man and mammals complete continuity from segment to segment, and 

 not even a sign of constriction between them. Each nerve is attached to the surface 

 of the cord by two roots, one of which is anterior or ventral and non-gangliated, 1 the 

 other is posterior or dorsal and is provided with a ganglion. The uppermost two 

 or three nerve-roots cross the subarachnoid and subdural spaces nearly horizontally 

 (figs. 1 and 5), but the rest pass across with a more and more oblique downward 

 inclination until their direction is almost vertical, and indeed the lower part of the 

 theca below the termination of the cord (fig. 2, D, E), is occupied by the descending 

 roots of the lumbar and sacral nerves, passing to the foramina between the corre- 

 sponding vertebrae. This mass of nerve-roots, which conceals the delicate fihun 

 terminale, is named the cauda equina (figs. 4, 5, 6). 



The relation between the spines of the vertebras and the places of attachment of 

 the nerve-roots to the cord is illustrated by the appended diagram (fig. 3) from 

 Reid, which is founded upon observations made on six adult subjects. From this it 

 will be seen that there is a much larger amount of variation than might have been 

 supposed. This is especially the case with the dorsal nerve-roots, some of which 

 show variations of their position of origin extending over a distance covered by as 

 many as three spinous processes. Certain general facts can, however, be made out 

 which are not without practical interest. Amongst these are the position of the 

 second cervical nerve opposite the arch of the atlas ; that of the first dorsal or 

 thoracic nerve, opposite the sixth or seventh cervical spine ; that of the seventh 

 thoracic nerve, opposite the fourth or fifth dorsal spines, and of the sacral nerves, the 

 range of which extends from the eleventh dorsal to the first lumbar spine. The 

 line of origin of the sacral nerves very nearly corresponds in vertical extent with the 

 body of the first lumbar vertebra. 



No doubt this variation is largely accounted for by the variations in length and 

 obliquity of the spinous processes of the vertebrae, and accordingly we find that 

 there is least fluctuation of relative position at the top and bottom of the series. 

 The anterior and posterior (ventral and dorsal) nerve-roots belonging to the same 

 segment of the cord leave it practically at the same level (Reid). 



The cervical enlargement (see next page) about corresponds in vertical extent 

 with the spines of the cervical vertebras, while the lumbar enlargement corresponds 

 with the spines of the tenth, eleventh, and twelfth thoracic and the interval between 

 the last named and the first lumbar. 



In section the cord is nearly circular, especially in the thoracic region, but it is 

 somewhat flattened before and behind. In the thoracic region, it measures about 

 ten millimeters (0'4 inch) from side to side, and about eight from before back. The 



1 Some animals (e.g. , cat) have a few ganglion-cells interpolated amongst the fibres of the anterior or 

 ventral nerve-roots. Hoche finds that in the anterior roots of the lower lumbar and sacral nerves of 

 man, just at their junction with the cord, ganglion-cells, like those of the posterior root, are almost 

 constantly present, lying singly or in groups, and connected with some of the issuing nerve-fibres by a 

 T-shaped junction. 



B 2 



