242 FUNCTIONS OF THE NERVOUS SYSTEM. 



with some modification, embody the general principles ascertained by ex- 

 periment, respecting the transmission of sensory and motor impressions. 

 Their rationale will be at once understood, from the facts already mentioned 

 in regard to the isolated characters of each fibril, and the identity of its 

 endowments through its whole course, i. When the whole trunk of a sensory 

 nerve is irritated, a sensation is produced, which is referred by the mind to 

 the parts to which its branches are ultimately distributed ; and if only part of 

 the trunk be irritated, the sensation will be referred to those parts only, which 

 are supplied by the fibrils it contains. This is evidently caused by the pro- 

 duction of a change in the sensorium, corresponding with that which would 

 have been transmitted from the peripheral origins of the nerves, had the 

 impression been made upon them. Such a change only requires the integrity 

 of the afferent trunk, between the point irritated and the sensorium ; and is 

 not at all dependent upon the state of the extremity, to which the sensations 

 are referred : for this may have been paralyzed by the division of the nerve ; 

 or altogether separated, as in amputation; or the relative position of its parts 

 may have been changed. It results from the foregoing, that, when different 

 parts of the thickness of the same trunk are separately subjected to irritation, 

 the sensations are successively referred to the several parts supplied by these 

 divisions. This may be easily shown by compressing the ulnar nerve, in 

 different directions, where it passes at the inner side of the elbow-joint. 



ii. The sensation produced by irritation of a branch of the nerve, is con- 

 fined to the parts to which that branch is distributed, and does not affect the 

 branches which come off from the nerve higher up. The rationale of this 

 law is at once understood: but it should be mentioned that there are certain 

 conditions, in which the irritation of a single nerve will give rise to sensations 

 over a great extent of the body. This seems due, however, to a particular 

 state of the central organs ; and not to any direct communication among the 

 sensory fibres. 



in. The motor influence is propagated only in a centrifugal direction, never 

 in a retrograde course. It may originate in a spontaneous change in the 

 central organs : or it may be excited by an impression conveyed to them 

 through afferent nerves ; but in both cases its law is the same. 



iv. When the whole trunk of a motor nerve is irritated, all the muscles 

 which it supplies are caused to contract: but when only a part of the trunk 

 or a branch is irritated, the contraction is confined to the muscles, which 

 receive their nervous fibres from it. This contraction evidently results from 

 the similarity between the effect of an artificial stimulus applied to the trunk 

 in its course, and that of the change in the central organs by which the motor 

 influence is ordinarily propagated. In this instance, as in the other, there is 

 no lateral communication between the fibrils. 



300. Various methods of determining the functions of particular nerves 

 present themselves to the Physiological inquirer. One source of evidence is 

 drawn from their anatomical distribution. For example, if a nervous trunk 

 is found to lose itself entirely in the substance of muscles, it may be inferred 

 to be chiefly, if not entirely, motor or efferent. In this manner, Willis long 

 ago determined that the third, fourth, sixth, portio dura of the seventh, and 

 ninth cranial nerves, are almost entirely subservient to muscular movement; 

 and the same had been observed of the fibres proceeding from the small root 

 of the fifth pair, before Sir C. Bell experimentally determined the double 

 function of that division of the nerve, into which alone it enters. Again, 

 where a nerve passes through the muscles, with little or no ramification among 

 them, and proceeds to a cutaneous or mucous surface, on which its branches 

 are minutely distributed, there is equal reason to believe that it is of a sensory, 

 or rather of an afferent, character. In this manner Willis came to the con- 



