NERVOUS IRRITABILITY. 361 



sibility in particular regions of the body, which disappears after a 

 short time, while other symptoms, immediately dependent on the 

 wound, are persistent;* and according to L'Etievant,f section of one 

 sensitive nerve may suspend, for a time, the sensibility of neighbor- 

 ing fibres derived from other nerves. 



The irritability of sensitive nerve fibres may also be abnormally 

 increased by vascular congestion or Ideal injuries. The application 

 of cold, or shutting off the supply of blood by the ligature of arteries, 

 may produce in the nerve, before it reaches the stage of insensibility, a 

 condition of unnatural excitement, indicated by pain in the parts cor- 

 responding to its distribution. 



During life the irritability of sensitive nerves is manifested by con- 

 scious sensation. After death it may be shown to exist, for a certain 

 time, by reflex actions taking place in the spinal cord and other parts 

 of the nervous system. 



Irritability of Motor Fibres. 



The motor nerves are especially adapted for the study of nervous 

 irritability, because their excitement produces a visible muscular con- 

 traction ; and this may take place, after both the nerve and its muscle 

 have been separated from the body. But to produce this result, as in 

 the case of the sensitive nerves, two conditions are requisite, namely ; 

 first, the nerve fibres must preserve their irritability ; and secondly, 

 the muscle must be capable of responding to the nervous stimulus. 

 These two conditions may therefore be studied in con- 

 nection with each other. 



Muscular Irritability. This is best shown in the 

 cold-blooded animals, in which it continues longer than 

 in birds or mammalians. If a frog's leg be separated 

 from the body, the skin removed, and the poles of a 

 galvano-electric apparatus (Fig. 94, a, b) applied to the 

 denuded muscles, a contraction takes place each time 

 the circuit is completed. In this case, the electric 

 stimulus acts directly on the muscles, and thus calls 

 out their contractility. A single muscular fibre, placed 

 under the microscope, may sometimes be seen to con- 

 tract for a certain time after its separation from the 

 neighboring parts. The muscles may also be excited 

 by pinching, pricking, the contact of hot or cold bodies, 

 or the application of acid, alkaline, or saline solutions. the poles of a^i- 

 But the most efficient and manageable stimulus is the vanic battery ap- 

 electric discharge Jj 1 ** the muscles 



Nervous Irritability. To exhibit the irritability of 

 motor nerve fibres, a frog's leg is prepared, as in the preceding experi- 

 ment, except that a portion of the sciatic nerve is retained in connection 



* Veyssi&re Recherches sur PHemianaesthesie. Paris, 1874, p. 78. 

 f Traite des Sections Nerveuses. Paris, 1873, pp. 171, 192. 



