302 



ADVANCED PHYSIOLOGY 



on its anterior and one on its posterior surface, the anterior 

 being more open and more shallow than the posterior. The 

 two halves, which are clearly shown in Figure 153, are held 

 together by a central connecting portion, about one third of 

 the diameter in width. 



The grooves of the cord are still better appreciated by a 

 study of a cross section; Fig. 154. Such a section, too, shows 

 that the cord, like the brain, is made up of two kinds of 

 material, nerve fibres and nerve cells, though in reversed re- 

 lations, the outer layers of the cord being of white, fibrous 

 matter, and the inner of gray, cellular matter. Recognizing 

 that the nerve fibres simply conduct impulses, while the nerve 

 cells have other more complex functions, it will be evident 

 that the cord has these two different classes of activities. 

 Since the process of conduction is the simpler matter, we shall 

 study it first. 



The Cord as a Conductor of Impulses. Messages sent 



through the cord pass 



Pojferior Roof- either up or down in the 



white matter; but do im- 

 pulses going up the cord 

 follow the same paths as 

 those going down? This 

 question has been answered 

 by experiments on some of 

 the lower animals which are 

 constructed essentially like 

 man, and also by observa- 

 tion of the results in human 

 beings in which the cord is 

 diseased or has been injured 

 by accident. These observa- 

 tions have shown that if a 

 part of the cord is disabled, sometimes sensation and some- 



Ascendmg 



i \ \ ' 



Root 



Descending 



FIG. 155. DIAGRAM OB^ A CROSS 

 SECTION OF THE SPINAL CORD 



Showing the parts that carry messages 

 up (ascending) and those that carry 

 messages down the cord (descending). 

 (Modified from Flint and Landois) 



