78 THE NERVOUS SYSTEM 



At the same time the roots of the lumbar and sacral nerves become greatly 

 elongated. They run in a caudal direction from their origin to the same inter- 

 vertebral foramina through which they made their exit before the cord shifted 

 its position. Since the thoracic portion of the cord has changed its relative 

 position but little, and the cervical part even less, the cervical roots run almost 

 directly lateralward, while those of the thoracic nerves incline but little in a 

 caudal direction. 



Since the spinal cord ends opposite the first or second lumbar vertebra, the 

 roots of the lumbar, sacral, and coccygeal nerves, in order to reach their proper 

 intervertebral foramina, descend vertically in the canal around the conus medul- 

 laris and filum terminale. In this way there is formed a large bundle, which is 

 composed of the roots of all the spinal nerves below the first lumbar and has 

 been given the very descriptive name cauda equina. 



The amount of relative shortening of the various segments of the cord differs 

 in different individuals. In Fig. 53, where the quadrilateral areas represent the 

 bodies of the vertebrae, we have indicated the average position of each segment 

 of the spinal cord. This figure is based on data published by Reid (1889). It is 

 obvious that the segments are longer in the thoracic than in the cervical and 

 lumbar portions of the cord, while the sacral segments are even shorter (see 

 also Fig. 59). 



We have been at some pains to explain the development of the cauda equina 

 and the vertebral level of the various segments of the spinal cord because these 

 are matters of much practical importance. In spinal puncture the needle is 

 made to enter the subdural space caudal to the termination of the cord. In 

 locating lesions of the spinal cord it is necessary to know the position of its 

 various segments with reference to the vertebras. It is particularly important 

 to be able to distinguish between an injury to the lower part of the spinal cord 

 and one which involves only the nerve roots in the cauda equina, since, although 

 the symptoms in the two cases may be nearly identical, damage to the spinal 

 cord is irreparable, while the nerve roots will regenerate. 



The Spinal Cord in Section. When a section is made through any part of 

 the brain or spinal cord one sees at once that they are composed of two kinds 

 of tissue the one whitish in color, the other gray, tinged with pink. The white 

 substance consists chiefly of myelinated fibers, the gray is made up of nerve- 

 cells, dendrites, unmyelinated and myelinated fibers, and many blood-vessels. 

 Both have a supporting framework of neuroglia. 



The gray substance (substantia grisea) of the spinal cord is centrally placed 



