CHAP, i.] THE SPINAL CORD. 677 



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 011 in an afferent nerve by the effects 

 which stimulation of the nerve produces in some central ner- 

 vous 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 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 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 im- 

 pulses, 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 senses ; but, on the other hand, a 

 knowledge of the central nervous system is necessary to a 

 proper understanding of the special senses ; and on the whole 

 it will be more convenient to study the former before the latter. 



450. The proof that the afferent and efferent fibres 

 which are both present in the trunk of a spinal nerve are 

 parted at the roots, the efferent fibres running exclusively in 



