420 NERVOUS IRRITABILITY 



bility of the nerve, like that of the muscles, depends directly upon its 

 anatomical structure and constitution; and so long as these remain 

 unimpaired, the nerve will retain its vital properties, though respira- 

 tion and circulation may have ceased. For the same reason, nervous 

 irritability lasts longer after death in the cold-blooded than in the 

 warm-blooded animals. Yarious artificial irritants may be employed 

 to call it into activity. Pinching or pricking the exposed nerve with 

 steel instruments, the application of caustic liquids, and the passage of 

 galvanic discharges, all have this effect. The galvanic current, how- 

 ever, is the best means to employ for this purpose, since it is more 

 delicate in its operation than the others, and will continue to succeed 

 for a longer time. 



The nerve is so sensitive to the galvanic current that it will respond 

 to it when insensible to all other kinds of stimulus. A frog's leg 

 freshly prepared, as above, with the nerve attached, will react so readity 

 when a discharge is passed through the nerve, that it forms an ex- 

 tremely delicate instrument for detecting the presence of electric cur- 

 rents of low intensity, and has been sometimes used for this purpose 

 under the name of the " galvanoscopic frog." It is only necessary to 

 introduce the nerve as part of the electric circuit ; and if even a very 

 feeble current be present, it is at once betrayed by a muscular con- 

 traction. 



Nervous irritability, like that of the muscles, is exhausted by repeated 

 excitement. If a frog's leg, prepared in the manner above described, 

 with the sciatic nerve attached, be allowed to remain at rest in a damp 

 and cool place, where its tissue will not become altered by desiccation, 

 the nerve will remain irritable for many hours ; but if it be excited, 

 soon after its separation from the body, by repeated shocks, it begins 

 to react with diminished energy, and becomes gradually less irritable, 

 until at last it ceases to exhibit any further excitability. If it be now 

 allowed to remain for a time at rest, its irritability will be partially 

 restored ; and muscular contraction will again ensue on the application 

 of a stimulus to the nerve. Exhausted a second time, and a second 

 time allowed to repose, it will again recover itself; and this may be 

 repeated several times in succession. At each repetition, however, the 

 recovery of nervous irritability is less complete, until it finally disap- 

 pears altogether, and can no longer be recalled. The irritability of the 

 muscles may be exhausted, in a similar way, by repeated excitement. 



Yarious circumstances tend to diminish or suspend the irritabilit}^ of 

 the motor nerve fibres. As in the case of the sensitive fibres, compres- 

 sion, cold, or other similar agencies will depress the power of the 

 muscular nerves, so that they can no longer excite a contraction when 

 subjected to the galvanic current. Severe and sudden mechanical in- 

 juries often have the same effect ; as where general muscular relaxation, 

 or diminished power of voluntary motion, is produced by any extensive 

 contusion or laceration of one of the limbs. Such an injury produces a 

 general disturbance or shock^ which affects the entire nervous system, 



