556 THE NERVOUS SYSTEM 



effect. It must be remembered, however, that in the anterior or efferent 

 nerves other fibers besides motor are contained, e.g., vaso-motor, secretory, 

 heat fibers, and when the distal end of a divided nerve is stimulated, the 

 effects are exercised not only upon muscles, but upon glands, blood vessels, 

 etc. Stimulation of the distal portions of the divided posterior roots, on the 

 other hand, produces no muscular movements and no manifestations of 

 pain; for, as already stated, sensory nerves convey impressions only toward 

 the nerve centers. Stimulation of the proximal portions of these roots elicits 

 signs of intense pain. Muscular movements also ensue; but these are the 

 result of the reflex stimulation of the motor neurones of the anterior horn 

 of the cord or are movements in response to the afferent impulses passing 

 to higher centers from the roots stimulated. 



Functions of the Ganglia on Posterior Roots. The cells of the posterior 

 ganglia act as centers for the nutrition of the nerve fibers given off from them. 

 When these are cut, the parts of the nerves so severed degenerate, while the 

 parts which remain in connection with the cells of the ganglia do not. Thus 

 on section of the posterior nerve root beyond the ganglia the peripheral part 

 degenerates and the central does not, and on section of the root between 

 the ganglion and the cord the central part degenerates and the peripheral is 

 unaffected. The number of nerve cells in the spinal ganglia far exceeds 

 the number of nerve fibers in the corresponding root (Hardesty). The extra 

 cells, see figure 417, serve at least in part as association neurones connecting 

 the bipolar cells of the ganglion, cells of Dogiel, and to connect the ganglion 

 with the sympathetic system of nerves. 



Spinal Reflexes in Man and Mammals. Much of our knowledge of 

 the reflexes of the cord is derived from experiments on dogs, though paral- 

 ysis of the lower extremities in man, by accident or otherwise, has given con- 

 firmatory information. In man the spinal cord is so much under the control 

 of the higher nerve centers that its own individual functions in relation to re- 

 flex action are apt to be overlooked. But if the skin of the foot is stimulated, 

 in a man whose lumbar cord is completely separated by injury or disease, the 

 foot will be drawn away from the stimulus; or, if the stimulus be strong 

 enough, the entire leg w r ill be moved. In both cases the movement may be 

 orderly and well co-ordinated, and shows that the sensory stimulus has pro- 

 duced a co-ordinated reflex through the lumbar cord. The injured person 

 feels no sensation of pain nor of action, and the phenomenon is independent 

 of the higher nerve regions. The stimulus that is supplied to man must be 

 carefully graded, since when too intense it calls forth muscular spasms or 

 convulsive action. 



When the cord of mammals is first cut, the shock is very great, and the 

 lower or isolated portion of the cord remains for a time quite non-irritable. 

 The vaso-motor and thermogenic centers are cut off from the periphery so 

 that there is great vascular dilatation and marked fall of temperature, the 



