510 THE NERVOUS SYSTEM 



II. THE STRUCTURE AND FUNCTION OF THE 

 SPINAL CORD. 



STRUCTURE. 



The spinal cord is a cylindrical column of nerve-substance connected above 

 with the brain through the medium of the bulb, and terminating below in a 

 slender filament of nerve substance, the filum terminate, which lies in the midst 

 of the roots of the many nerves forming the cauda equina. 



General Features. The cord is composed of nerve fibers and nerve 

 cells. The former are situated externally and constitute the chief portion 

 of the cord, while the latter occupy its central or axial portion and are so 

 disposed that on the surface of a transverse section of the cord two somewhat 

 crescentic grayish masses connected by a narrower portion or isthmus 

 appear, figure 358. Passing through the center of the cord in a longitudinal 

 direction is a minute canal, the central canal, which is continued through the 

 whole length of the cord, opening above into the space at the back of the 

 medulla oblongata and pons Varolii called the fourth ventricle. The canal 

 is lined by a layer of columnar ciliated epithelium. 



The spinal cord consists of exactly symmetrical halves, separated anteriorly 

 and posteriorly by vertical fissures (the posterior fissure being deeper but less 

 wide and distinct than the anterior), and united in the middle by nervous 

 matter which forms the commissures. The central part, which contains the 

 central canal, is known as the gray commissure, and is bounded by the anterior 

 white commissure in front, and the posterior white commissure behind. Each 

 half of the spinal cord is marked on the sides (obscurely at the lower part, but 

 distinctly above) by two longitudinal furrows, which divide it into three por- 

 tions, columns, or tracts an anterior, lateral, and posterior. From the groove 

 between the anterior and lateral columns spring the anterior roots of the 

 spinal nerves; and just in front of the groove between the lateral and 

 posterior columns arise the posterior roots of the same; a pair of roots on each 

 side corresponding to each vertebra. 



The nerve tracts of the cord are made up of medullated nerve fibers of 

 different sizes, arranged longitudinally, and of a supporting material of ordi- 

 nary fibrous connective tissue and neuroglia, figure 105. 



The general rule respecting the size of different segments of the cord appears 

 to be that each is in direct proportion in this respect to the size and number of 

 nerve roots given off from it, and has but little relation to the size or number 

 of those given off below it. Thus the cord is very large in the middle and 

 lower part of its cervical portion, whence arise the large nerve roots for the 

 formation of the brachial plexuses and the nerve supply of the upper extrem- 

 ities; and again enlarges at the lowest part of its dorsal portion and the upper 



