THE NERVOUS SYSTEM. 571 



into the anterior roots. The sensory neurons have their cells or start- 

 ing-points in the posterior spinal ganglia, these being large gang- 

 liouic masses which lie upon the posterior roots. These cells have 

 a process which runs spineward through the posterior roots into the 

 spinal cord, and another which runs peripherally, forming the sen- 

 sory nerve. The intermediate neurons have their cells of origin in the 

 posterior horns and median part of the gray matter, and, to a slight ex- 

 tent, in the anterior horns. Their cells form the intrinsic cells of the 

 spinal cord, and also assist in the conduction of sensory and other affer- 

 ent impulses. For example, the neurons, starting with the cells lying 

 in Clarke's column, send their processes up into the cerebellum, and 

 thus continue afferent impulses brought to the neurons through the pos- 

 terior roots. On the other hand, other groups of cells lie in the lateral 

 part of the gray matter and give rise to processes which pass out into the 

 lateral columns and then enter the gray matter again, to connect with 

 cells at different levels. These are the intermediate neurons which are 

 cornmissural in their functions. 



FUNCTIONS OF THE SPINAL NERVE-KOOTS. 



The anterior spinal nerve-roots are efferent in function: the posterior 

 are afferent. The fact is proved in various ways. Division of the 

 anterior roots of one or more nerves is followed by complete loss of mo- 

 tion in the parts supplied by the fibres of such roots ; but the sensation 

 of the same parts remains perfect. Division of the posterior roots 

 destroys the sensibility of the parts supplied by their fibres, while the 

 power of motion continues unimpaired. Moreover, irritation of the 

 ends of the distal portions of the divided anterior roots of a nerve excites 

 muscular movements; irritation of the ends of the proximal portions, 

 which are still in connection with the cord, is followed by no appreciable 

 eff-ect. It must be remembered, however, that in the anterior or efferent 

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

 heat fibres, and it may be supposed that when the distal end of a divided 

 nerve is stimulated, the effects would be exercised not only upon mus- 

 cles, but upon glands, blood-vessels, etc. Irritation 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, sen- 

 sory nerves convey impressions only toward the nervous centres : but 

 irritation of the proximal portions of these roots elicits signs of intense 

 suffering. Occasionally, under this last irritation, muscular movements 

 also ensue; but these are either voluntary, or the result of the irritation 

 being reflected from the sensory to the motor fibres. Occasionally, too, 

 irritation of the distal ends of divided anterior roots elicits signs of pain, 



