390 OF NERVOUS IRRITABILITY 



removed, and the poles of a galvanic apparatus applied to the sur- 

 face of the muscle (a, 1)), a contraction takes place every time the 

 circuit is completed and a discharge passed through the tissues of 

 the limb. The leg of the frog, prepared in this way, may be em- 

 ployed for a long time for the purpose of exhibiting the effect of 

 various kinds of stimulus upon the muscles. All the mechanical 

 and chemical irritants which we have mentioned, pricking, pinching, 

 cauterization, galvanism, &o v act with more or less energy and 

 promptitude, though the most efficient of all is the electric discharge. 



Continued irritation exhausts the irritability of the muscles. It is 

 found that the irritability of the muscles wears out after death more 

 rapidly if they be artificially excited, than if they be allowed to 

 remain at rest. During life, the only habitual excitant of mus- 

 cular contraction is the peculiar stimulus conveyed by the nerves. 

 After death this stimulus may be replaced or imitated, to a certain 

 extent, by other irritants ; but their application gradually exhausts 

 the contractility of the muscle and hastens its final disappearance. 

 Under ordinary circumstances, the post-mortem irritability of the 

 muscle remains until the commencement of cadaveric rigidity. 

 When this has become fairly established, the muscles will no longer 

 contract under the application of an artificial stimulus. 



Certain poisonous substances have the power of destroying the 

 irritability of the muscles by a direct action upon their tissue. 

 Sulphocyanide of potassium, for example, introduced into the cir- 

 culation in sufficient quantity to cause death, destroys entirely the 

 muscular irritability, so that no contraction can afterward be pro- 

 duced by the application of an external stimulant. 



Nervous Irritability. The irritability of the nerves is the pro- 

 perty by which they may be excited by an external stimulus, so as 

 to be called into activity and excite in their turn other organs to 

 which their filaments are distributed. When a nerve is irritated, 

 therefore, its power of reaction, or its irritability, can only be esti- 

 mated by the degree of excitement produced in the organ to which 

 the nerve is distributed. A nerve running from the integument to 

 the brain produces, when irritated, a painful sensation; one dis- 

 tributed to a glandular organ produces increased secretion ; one dis- 

 tributed to a muscle produces contraction. Of all these effects, 

 muscular contraction is found to be the best test and measure of 

 nervous irritability, for purposes of experiment. Sensation cannot 

 of course be relied on for this purpose, since both consciousness and 

 volition are abolished at the time of death. The activity of the 



