570 HANDBOOK OF PHYSIOLOGY. 



laterals with the intrinsic cells of the gray matter at different levels of 

 the cord. One can realize that each nerve-root has, in this way, an 

 effective grip upon a large extent of the cord. This is seen well by 

 studying figs. 352A and 353. 



The Peculiarities of different regions of the Spinal Cord. The outline of the 

 gray matter and the relative proportion of the white matter varies in different 

 regions of the spinal cord, and it is, therefore, possible to tell approximately 

 from what region any given transverse section of the spinal cord has been 

 taken. The white matter increases in amount from below upward. The 

 amount of gray matter varies ; it is greatest in the cervical and lumbar enlarge- 

 ments, viz. , at and about the 5th lumbar and 6th cervical nerve, and least in the 

 thoracic region. The greatest development of gray matter corresponds with 

 greatest number of nerve-fibres passing from the cord. 



In the cervical enlargement the gray matter occupies a large proportion of the 

 section, the gray commissure is short and thick, the anterior horn is blunt, while 

 the posterior is somewhat tapering. The anterior and posterior roots run some 

 distance through the white matter before they reach the periphery. 



In the dorsal region the gray matter bears only a small relation to the white, 

 and the posterior roots in particular run a long course through the white matter 

 before they leave the cord; the gray commissure is thinner and narrower than 

 in the cervical region. The tractus intermedio-lateralis is here most marked. 



In the lumbar enlargement the gray matter again bears a very large propor- 

 tion to the whole size of the transverse section, but its posterior cornua are 

 shorter and blunter than they are in the cervical region. The gray commis- 

 sure is short and extremely narrow. 



At the upper part of the conus medullaris, which is the portion of the cord 

 immediately below the lumbar enlargement, the gray substance occupies 

 nearly the whole of the transverse section, as it is only invested by a thin 

 layer of white substance. This thin layer is wanting in the neighborhood of 

 the posterior nerve-roots. The great commissure is extremely thick. 



At the level of the fifth sacral vertebra the gray matter is again in excess, and 

 the central canal is enlarged, appearing T-shaped in section ; while in the 

 upper portion of the filum terminate the gray matter is uniform in shape without 

 any central canal. 



The shape of the cord changes from the sacral and lumbar region 

 where it is circular to the thoracic where it is oval, and to the cervical 

 where the lateral diameter considerably exceeds the antero-posterior; 

 the change in shape is due to a gradual increase of the lateral columns. 

 THE SPINAL CORD AND NERVE-ROOTS A MASS OF NERVE-UNITS. 

 We have, in the foregoing, described the spinal cord as being composed 

 of white and gray matter, and these substances, in tarn, being composed 

 of nerve-fibres and nerve-cells, and a supporting substance called neurog- 

 lia. From the physiologist's point of view, the spinal cord is considered 

 to be composed of a mass of nerve-units or neurons. These are divided 

 into three great classes: the motor neurons, the sensory neurons, and 

 the intermediate neurons. The motor neurons make up the larger part 

 of the nerve-tissue in the anterior horns; their neuraxons pass out 



