234 THE NERVOUS SYSTEM 



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

 However, only the message traveling in the physiological direc- 

 tion is manifest, for it is the only one which finds a suitable 

 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, special sensory, 

 vaso-motor, motor, trophic, secretory but the presence of all 

 these does not interfere with the individuality 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 im- 

 pressions 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 impressions 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 -fa of a second. The recognition of a simple 

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

 quires about 2^ of a second. Furthermore, the part played by 

 the spinal cord in reflex action (to be considered later) also con- 

 sumes an appreciable period; this is found to be more than 

 twelve times the period occupied in the transmission 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, are much 

 less easily managed and regulated than is electricity. This 

 agent may be applied time after time to a nerve trunk without 

 causing any permanent change in its conductivity, and the 



