ANESTHESIA OF THE CUTANEOUS NERVES. 345 



SYMPTOMS AISTD COURSE. Anaesthesia is either complete or in- 

 complete. In the first case, irritants acting from without, especially 

 pressure and change of temperature, produce no sensation whatever. 

 In the second case, though an impression is produced, it is indistinct 

 and inaccurate. Where the anaesthesia is incomplete, there is often a 

 numb or muffled sensation. The patient feels as though a foreign body 

 lay between his skin and the object which he actually touches. HenU 

 believes that this state of affairs is due to an extinction of sensibility of 

 the peripheral terminations of the nerves, and a retrocession of the 

 sensibility in the direction of the centre, so that there actually is a 

 layer of passive substance between the nerve and the object touched. 

 Upon this hypothesis, it is very easy to explain why cold, whose im- 

 mediate action is upon the tips of the sensory nerves, should make the 

 skin feel numb and muffled. There is a peculiar form of anaesthesia 

 called analgesia. In these cases there is no pain, even under the most 

 intense irritation of the nerve, although the sense of touch is not im- 

 paired. We have no satisfactory means of accounting for analgesia. 

 With Henle, we might suppose it to be a low grade of anaesthesia, 

 and might assume that excessive irritation, instead of producing the 

 usual amount of perception, namely pain, merely gives rise to that 

 normally induced by moderate stimulation of the nerve, were it not 

 that, under such a condition, moderate irritation should be impercepti- 

 ble, which is not the case in analgesia. The modifications of anaes- 

 thesia, known as partial losses of sensation, are still more obscure. 

 Here the effect of certain irritants is extinguished, while that of other 

 irritants of no greater power continues. For instance, there may be a 

 loss of the sense of touch, with unimpaired sense of temperature, or 

 conversely. 



The so-called " law of excentric impression," according to which 

 the excitement of a sensory nerve, even though produced by irritation 

 of its trunk, always produces the impression of an excitement of its 

 peripheral extremity, fully explains why patients often suffer pain in 

 regions which are entirely insensible to external irritation, a phenome- 

 non to which the name of ancesthesia dolorosa has been applied. 

 Anaesthesia dolorosa arises, first, in all cases in which a nerve has lost 

 its sensitiveness throughout a certain portion of its length, while, at 

 the same time, a severe irritation is made to act, above this point, upon 

 that portion of the nerve which still remains in connection with the 

 brain. It is easy to see that both of these factors often coexist, as 

 when a tumor presses upon a nerve. Here the tract of nerve, lying 

 between the compressing growth and the brain, remains excitable and 

 receptive of lasting impressions, while from the tumor to the periphery 

 ihe nerve has lost its irritability. 



