CHAP, i.] THE SPINAL CORD. 851 



separated from its muscle ( 72) 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 

 differences are capable of exact measurement. Secondly, when 

 we apply a stimulus 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 labours 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 modifying reflex action, or 

 modifying automatic action, or affecting consciousness, we are 

 met on the very threshold of every enquiry 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 

 shewn 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. 



We shall further see in detail later on that our consciousness 

 may be affected in many different ways by afferent impulses; 

 we must distinguish not only sensory from other afferent impulses, 

 but also different kinds of sensory impulses from each other. 

 Certain afferent nerves are spoken of as nerves of special sense, 

 and the nature of the afferent impulses passing along these special 

 nerves together with the modifications of consciousness caused by 

 arrival of these impulses at the central nervous system constitute 

 by themselves a complex and difficult branch of study. In some 

 of the problems connected with the central nervous system we 

 shall have to appeal to the results of a study of these special 



