608 THE FUNCTION OF THE SPINAL CORD 



to 135/x. They are efferent in their nature and innervate the skeletal 

 musculature. Second in importance are the somewhat smaller cells 

 of the lateral horn, the axons of which leave the cord by way of the 

 anterior roots but finally separate to enter the sympathetic ganglia. 

 In this way, the white ramus communicans is formed, constituting 

 one of the efferent bridges between the cerebrospinal and sympathetic 

 systems. As has been stated above, the afferent cells of the cord are 

 contained in the spinal ganglia which are situated upon the different 

 posterior roots. Other afferent cells of the projection system form the 

 nucleus eracilis and cuneatus, the end-stations of the posterior fasciculi. 



FIG. 300. SPINAL GANGLION-CELLS SHOWING TRANSITION FROM BIPOLAR TO UNIPOLAR 



CONDITION. (Holmyren.) 



The Functional Basis of the White Matter The characteristic 

 appearance of the gray matter and white matter is dependent upon 

 certain structural differences. The former is composed principally 

 of cell-bodies and the dendrites and axons in their immediate vicinity, 

 while the latter consists chiefly of axons enveloped in their medullary 

 sheaths, in other words, of nerve fibers. It is evident that the white 

 matter decreases constantly in the direction toward the tip of the cord, 

 because the number of fibers still retained at its lumbar level is much 

 smaller than that near the medulla. Fibers leave this structure all 

 the time to reach peripheral parts, and fibers enter it continuously to 

 attain the higher centers. This does not imply, however, that there 

 is an absolute proportion between these fibers and the total area of the 

 white matter at different levels of the cord, because a large number of 

 them do not pass all the way through, but form merely local reflex 

 connections. In addition, it should be noted that the relative amounts 

 of gray and white matter vary at different levels of the cord, thereby 

 enabling us to determine with accuracy from what particular area any 

 given section has been taken. Sections from its lumbar region are 

 characterized by a copious amount of gray matter, while those from 

 its cervical portion are relatively poor in this substance. Besides, as 

 especially large numbers of fibers arise in its cervical and lumbar seg- 

 ments at the points of origin of the plexuses of the arms and legs, the 

 total cross-area of the cord must be markedly increased at these levels. 



The posterior roots serve as points of entrance for about half a 

 million fibers and we may assume that an equal number leaves by 

 way of the anterior roots. The afferent impulses which are in this 



