FUNCTIONS OF THE SPINAL COED. 



789 



The spinal cord is not only the path of conduction of nerve impres- 

 sions from the periphery of the brain and the reverse, but is also the 

 seat of a large number of nervous centres which are capable of acting 

 as reflex centres, or even of 

 originating impulses. 



The functions of the spinal 

 cord are, therefore, to be con- 

 sidered first, as a collection 

 of nerve-centres, and, second, 

 as a conductor of afferent and 

 efferent impulses. 



(a) The Spinal Cord as a 

 Collection of Nerve- Centres. 

 It has been already stated that 

 reflex action requires for its 

 performance afferent and effer- 

 ent nerve-fibres and a nerve- 

 centre, and the spinal cord has 

 been mentioned as the main 

 seat of the centres of reflex 

 action. 



When the spinal cord is 

 divided in an animal, the ap- 

 plication of a stimulus to its skin produces muscular movements of 

 the most diverse kinds, depending, as already indicated, upon the 

 nature, intensit}', and the locality of the stimulus. 



The histology of the spinal A 



cord indicates that, from the 

 direct communication of the 

 posterior roots (which have 

 been found to be paths of con- 

 duction of sensation) through 

 the gray commissure with the 

 anterior roots (which have been 

 found to be the paths of motor 

 impulses), afferent impulses 

 reach the spinal cord through 

 the sensory nerves and are 

 directty conducted to nerve- 

 centres, which again are in communication with motor nerves. It is, 

 therefore, evident that afferent impulses are brought directly to nerve- 

 cells, which again communicate the modified nerve impulse to motor 

 nerves (Fig. 336). In the spinal cord such centres of reflex action. may be 



FIG. 385. TRANSVERSE SECTION OF THE SPINAL 

 CORD IN THE DORSAL, REGION. AFTER 

 BEVAN LEWIS. (Yeo.) 



(For references see description under Fig. 333.) 



M 



FIG. 336. SECTION OF A SPINAL SEGMENT, 

 SHOWING A UNILATERAL AND CROSSED 

 REFLEX ACTION. (Landois.) 



A, anterior, and P, posterior surfaces ; M, muscle ; S, skin ; G, 

 ganglion. 



