234 THE NERVOUS SYSTEM 



trunk be stimulated at a given point, then the nerve impulse 

 can be demonstrated as passing away from the point of 

 stimulation in both directions" (American Text-book). 

 However, only the message traveling in the physiological di- 

 rection is manifest, for it is the only one which finds a suit- 

 able terminal. 



It is not to be concluded, however, that in any nerve 

 trunk, as the ulnar nerve, there may not be both afferent and 

 efferent fibers. Such, in fact, is the usual arrangement. Any 

 nerve trunk may contain all kinds of fibers sensory, spe- 

 cial sensory, vaso-motor, motor, trophic, secretory but the 

 presence of all these does not interfere with the individu- 

 ality and the individual action of each fiber. A nerve trunk 

 containing more than one kind of fibers is called a mixed 

 nerve. 



Speed of Nervous Conduction. It is stated that afferent 

 impressions are conveyed by nerves at the rate of about 120 

 feet per second; the rate for efferent impulses is somewhat 

 less rapid, probably no feet. In the spinal cord tactile im- 

 pressions are conveyed a little faster than in the nerves 

 proper, and painful impressions somewhat less than one- 

 half as fast. The rate of motor conduction in the cord is 

 said to be one-third the rate in the nerves. It has also been 

 demonstrated that an act of volition requires a definite 

 time for the inception of its performance; this is stated to 

 be about ^s of a second. The recognition of a simple im- 

 pression (conveyed in the opposite direction, of course) re- 

 quires about ^5 of a second. Furthermore, the part played 

 by the spinal cord in reflex action (to be considered later) 

 also consumes an appreciable period; this is found to be 

 more than twelve times the period occupied in the transmis- 

 sion of the impression to the cord or the impulse back to the 

 muscles. ' 



Action of Electricity Upon Nerves. A nerve may be irri- 

 tated in any one of several ways; but mechanical, thermal 

 and chemical irritants, besides working injury to the tissues, 



