526 THE SPINAL CORD. 



vous system to peripheral organs. Most efferent nerve-fibres carry impulses 

 to muscles, striated or plain, and the impulses passing along them give rise 

 to movements ; hence they are frequently spoken of as " motor " fibres. But 

 all efferent fibres do not end in or carry impulses to muscular fibres ; we have 

 seen for instance that some efferent fibres are secretory. Moreover, all the 

 nerve-fibres going to muscular fibres do not serve to produce movement ; 

 some of them, as in the case of certain vagus fibres going to the heart, are 

 inhibitory and may serve to stop movement. 



By " afferent " nerve-fibres we mean nerve fibres which in the body 

 usually carry impulses from peripheral organs to the central nervous sys- 

 tem. A very common effect of the arrival at the central nervous system of 

 impulses passing along afferent fibres is that change in consciousness which 

 we call a " sensation ;" hence afferent fibres or impulses are often called 

 " sensory " fibres or impulses. But as we have already in part seen, and as 

 we shall shortly see in greater detail, the central nervous system may be 

 affected by afferent impulses, and that in several ways, quite apart from the 

 development of any such change of consciousness as may be fairly called a 

 sensation. We shall see reason for thinking that afferent impulses reaching 

 the spinal cord, and, indeed, other parts of the central nervous system, may 

 modify reflex or automatic or other activity without necessarily giving rise 

 to a " sensation." Hence it is advisable to reserve the terms " efferent " and 

 " afferent " as more general modes of expression than " motor " or " sensory." 



We have seen in treating of muscle and nerve, that the changes pro- 

 duced in the muscle serve as our best guide for determining the changes 

 taking place in a motor nerve ; when a motor nerve is separated from its 

 muscle ( 70) the only change which we can appreciate in it is an electrical 

 change. Similarly in the case of an afferent nerve, the central system is our 

 chief teacher ; in a bundle of afferent fibres isolated from the central nervous 

 system, in a posterior root of a spinal nerve for instance, the only change 

 which we can appreciate is an electrical change. To learn the characters of 

 afferent impulses we must employ the central nervous system. But in this 

 we meet with difficulties. In studying the phenomena of motor nerves we 

 are greatly assisted by two facts. First, the muscular contraction by which 

 we judge of what is going on in the nerve is a comparatively simple thing, 

 one contraction differing from another only by such features as extent or 

 amount, duration, frequency of repetition and the like, and all such differ- 

 ences are capable of exact measurement. Secondly, when we apply a stim- 

 ulus directly to the nerve itself, the effects differ in degree only from those 

 which result when the nerve is set in action by natural stimuli, such as the 

 will. When we come, on the other hand, to investigate the phenomena of 

 afferent nerves, our labors are for the time rendered heavier, but in the end 

 more fruitful, by the following circumstances : First, when we judge of what 

 is going on in an afferent nerve by the effects which stimulation of the nerve 

 produces in some central nervous organ, in the way of exciting or modify- 

 ing reflex action, or modifying automatic action, or affecting consciousness, 

 we are met on the very threshold of every inquiry by the difficulty of clearly 

 distinguishing the events which belong exclusively to the afferent nerve from 

 those which belong to the central organ. Secondly, the effects of applying 

 a stimulus to the peripheral end-organ of an afferent nerve are very different 

 from those of applying the same stimulus directly to the nerve trunk. This 

 may be shown by the simple experience of comparing the sensation caused 

 by bringing any sharp body into contact with a nerve laid bare in a wound 

 with that caused by contact of an intact skin with the same body. These 

 and like differences reveal to us a complexity of impulses, of which the 

 phenomena of motor nerves gave us hardly a hint. 



