84 



THE PHYSIOLOGY OF MUSCLE AND NERVE. 



the motor fibers to the voluntary muscles emerge from the spinal 

 cord in the anterior roots, while the fibers that give rise to sensa- 

 tions enter the cord through the posterior roots (Fig. 29). These 

 facts have been demonstrated beyond all doubt. If an anterior 

 root is cut and the end attached to the cord is stimulated nothing 

 happens, but if the peripheral end is stimulated the muscles sup- 

 plied by that spinal nerve are thrown into contraction. If a 

 posterior root is cut and the peripheral end is stimulated no result 

 can be observed, while if the central stump attached to the cord is 

 stimulated pain (and reflex movements) are exhibited. Magendie 

 discovered an apparent exception in the phenomenon of recurrent 

 sensibility. When the anterior root is severed and its peripheral 

 end is stimulated only motor effects should be obtained. Magendie 

 observed, however, upon dogs that in certain cases the animals 



Fig. 29. — Diagram to show the origin of a spinal nerve from the cord by two roots: A, The 

 anterior or motor root; P, the posterior or sensory root; N, the spinal nerve; G, the posterior 

 root ganglion. 



showed signs of pain. This apparent exception to the general 

 rule was afterward explained satisfactorily. It was shown that 

 the fibers in question do not really belong to the anterior root, — 

 that is, they do not emerge from the cord with the root fibers; 

 they are, in fact, sensory fibers for the meningeal membranes of 

 the cord which are on their way to the posterior roots and which 

 enter the cord with the fibers of the latter. Since the work of 

 Bell and Magendie it has been a question whether their law applies 

 to all afferent and efferent fibers and not simply to the motor and 

 sensory fibers proper. The experimental evidence upon this point, 

 as far as the mammals are concerned, has accumulated slowly. 

 Various authors have shown that stimulation of the anterior roots 

 of certain spinal nerves may cause a constriction of the blood-ves- 

 sels, an erection of the hairs (stimulation of the pilomotor fibers), 

 a secretion of sweat, and so on, while stimulation of the posterior 



