GENERAL SUMMARY. 653 



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 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 successively irritated, the sensations are succes- 

 sively 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 sensa- 

 tions 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 fibre. 

 Thus, as it has been well remarked by Volkmann, " 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 irritation should be felt in sensations variously scattered about the line, 

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



II. 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 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. 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. But when only a part of the trunk or a branch is 

 irritated, the contraction is usually confined to the muscles which receive their 

 nervous fibres from it ; in this instance, as in the other, there is no lateral 

 communication between the fibrils. An exception exists, however, in regard 

 to galvanic irritation, which may be transmitted laterally when its ordinary 

 course is checked ; as has been shown by the following ingenious experiment 

 of M. du Bois-Reymond. If any motor nerve be selected which divari- 

 cates into two branches (as, for example, the sciatic nerve of a 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, 



1 " Kirkes and Paget's Handbook of Physiology," p. 375. It does not seem improba- 

 ble, however, that in the case of the compression or other irritation of a large nerve-trunk 

 the local pain may be produced through the instrumentality of nervi nervorum, the exist- 

 ence of which is scarcely less probable than that of vasa vasorum. 



