418 NERVOUS IRRITABILITY 



by the compression of a bandage or the application of cold, that no 

 stimulus applied to the part will produced any perceptible effect. Ac- 

 cording to the observations of Weir Mitchell, 1 the application of extreme 

 cold, in man, to the region of the ulnar nerve at the elbow produces, 

 when the chilling process has reached a certain stage, complete loss of 

 sensibility in the parts to which the nerve is distributed. The irrita- 

 bility of sensitive nerve fibres may also be temporarily suspended by 

 mechanical injuries in their immediate neighborhood, not involving the 

 fibres themselves. Thus a division of certain parts of the white sub- 

 stance in the brain or spinal cord, is known to produce a loss of sensi- 

 bility in particular regions of the body, which may disappear after a 

 short time, notwithstanding that the wounded fibres remain ununited ; 2 

 and according to the observations of L'Etievant, 3 section of one branch 

 of a sensitive nerve, beside the persistent anaesthesia of the divided 

 fibres, may also cause a temporary loss of sensibility in neighboring 

 fibres, derived by anastomosis from other branches. 



The irritability of sensitive nerve fibres may also be abnormally 

 increased by vascular congestion, or local 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 which is indicated by pain, in the 

 parts corresponding to its distribution. 



During life the irritability of sensitive nerves is manifested by the 

 evidences of conscious sensation. After death, as in a decapitated 

 animal, it may also be shown to exist, for a certain time, by the reflex 

 actions taking place in the spinal cord or in other parts of the nervous 

 system. 



Irritability of Motor Fibres. 



The motor nerves are especially convenient for studying the action 

 of nervous irritability, because their excitement has for its result a 

 visible muscular contraction ; and this may take place, even when the 

 nerve and its muscle have been separated from the rest of the body. 

 To produce this result, however, as in the case of the sensitive nerves, 

 two conditions are requisite, namely; first, the nerve fibre must pre- 

 serve its normal irritability ; and secondly, the muscular tissue must 

 also be capable of responding to a stimulus by the contraction of its 

 fibres. The laws regulating these two sets of phenomena may therefore 

 be studied in connection with each other. 



Mode of exhibiting Muscular Irritability. This is best shown in the 

 cold-blooded animals, since in them it continues active for a longer time 

 than in the birds and mammalians. A frog's leg is separated from the 

 body of the animal, the skin removed, and the poles of a galvano-electric 



1 Injuries of Nerves and their Consequences. Philadelphia, 1872, p. 59. 



2 Veyssiere, Recherches sur PHemianaesthe'sie. Paris, 1874, p. 78. 

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



