DETERMINATION OF THE FUNCTIONS OF NERVES. 573 



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 production of a change in th-e 

 sensorium, corresponding with that which would have been transmitted from 

 the peripheral organs of the nerves, had the impression been made upon, 

 them. Such a change only requires the integrity of the afferent trunk be- 

 t \\vrn the point irritated and the sensorium, and is not at all dependent upon 

 the state of the peripheral part to which the sensations are referred ; for this 

 may have been paralyzed by the division or other lesion of the nerve, or may 

 have been altogether" separated as in amputation, or the relative position of 

 its parts may have been changed as in autoplastic operations. So, when 

 different parts of the thickness of the same trunk are separately and succes- 

 sively irritated, 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. Still the mind undoubtedly does possess a certain power of 

 discriminating the part of the nerve-trunk on which the impression is made; 

 for whilst this impression is such as to produce sensations that are referred 

 to its peripheral extremities, pain is at the same time felt in the spot itself; 

 and it would seem as if slight impressions are only felt in the latter situation, 

 at least in the normal condition of the trunk or h'bre. Thus, as it has been 

 well remarked by Volkmanu, "if a needle's point be drawn in a straight 

 line across the back, or the thigh, or any part in which the nerves are widely 

 placed, the mind perceives the line of irritation as a straight one; whereas, 

 if it referred all impressions to the ends of irritated fibres, this mode of irri- 

 tation should be felt in sensations variously scattered about the line, at the 

 points where the uerve-fibres crossed by the needle terminate." 1 



ir. The sensation produced by irritation of a branch of the nerve is confined 

 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 intelligible; 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 "radiation of sensations " seems 

 rather due, however, to a particular state of the central organs, than to any 

 direct communication among the peripheral fibres. 



in. In general the motor influence is propagated only in a centrifugal di- 

 rection, and not 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. There 

 is, however, a well-known experiment termed the " paradoxical contraction 

 of muscle," which shows that motor nerves may under certain circumstances 

 convey impressions ceutripetally. If a motor nerve be selected which divides 

 into two branches (as for example the sciatic of the frog, which divides above 

 the bend of the knee into the tibial and peroneal branches), and a galvanic 

 stimulus be applied to either of these branches, this having been first divided 

 above its insertion into the muscles, the electrotonic state will be developed, 

 not merely in the portion of the trunk continuous with that branch, but also 

 in that which is continuous with the other branch, as will be made apparent 

 by the contraction in the muscles supplied by the latter. That this experi- 

 ment may be free from the possible fallacy resulting from the excitement of 

 reflex action, the trunk of the sciatic nerve should be divided high up, or 



1 Kirkes and Pagct's Handbook of Physiology, p. 37-5. 



