IRRITABILITY AND STIMULATION OF THE MUSCLE. 555 



Attention has repeatedly been directed to the analogies between muscle in 

 active contraction and in the state of rigidity- The form of the contracted and 

 of the rigid muscle is shortened and thickened; both are denser, of changed elas- 

 ticity, and evolve heat; the contents of the contracted as of the rigid muscle are 

 negative electrically as compared with resting or non-rigid contents; both evolve 

 free carbon dioxid and the remaining acid from the same source. A contraction 

 may, therefore, be regarded as a temporary rigidity, disappearing physiologically, 

 just as earlier investigators, and recently Bernstein, designated rigidity as being, 

 to a certain extent, the final vital act of the muscles. 



A muscle in process of becoming rigid will lift a weight, like a living, con- 

 tracting muscle. The height to which the weight is lifted by a rigid muscle is 

 greater in the case of small weights and less for heavy ones than if the living 

 muscle be stimulated to a maximum degree. If a muscle, in which heat-rigor has 

 been induced, be at first prevented from contracting 1 , and if later (for example 

 after ten minutes) it be set free, its elastic energy will cause it to contract, and 

 it must lose heat at the same time. 



The disappearance of cadaveric rigidity takes place at first as a result of 

 increased formation of acid in the muscle, by which the myosin is redissolved. 

 Subsequently, with the development of micro6rganisms putrefaction sets in, with 

 the associated evolution of ammonia, hydrogen sulphid, nitrogen, and carbon 

 dioxid. 



The loss of irritability in the muscles that precedes the onset of rigidity occurs 

 in the following order in man (beheaded criminal): Left ventricle, stomach, intes- 

 tine (fifty-five minutes), urinary bladder; right ventricle (sixty minutes); iris 

 (one hundred and five minutes) ; muscles of the face and the tongue (one hundred 

 and eighty minutes) ; the extensors of the extremities about one hour before the 

 flexors ; the muscles of the trunk (from five to six hours) . The esophagus remains 

 irritable for a long time. 



IRRITABILITY, STIMULATION, AND DEATH OF THE MUSCLE. 



By the irritability of a muscle is understood, its ability to contract in 

 response to stimuli applied directly to it (not to its nerves). Stimu- 

 lation is the state of functional activity in which a muscle is placed by 

 stimuli. At the moment of activity the stimulation causes the chemical 

 potential energy of the muscle to be converted into work and heat; 

 stimuli thus act as liberating forces. The mean temperature of the 

 body is most favorable for the manifestation of irritability. Each 

 muscle appears to possess a special degree of irritability peculiar to it- 

 self, as do likewise the nerves. 



So long as the current of blood in the muscle is uninterrupted, stimu- 

 lation first causes an increase in its functional activity, partly because 

 the circulation becomes more active in association with dilatation of the 

 vessels; later, however, the functional activity diminishes. 



This diminution in functional activity is a sign of fatigue. If the same stimu- 

 lation be continued, the muscular activity will exhibit a periodic variation, in 

 such manner that after a series of weaker contractions stronger ones will again 

 set in, followed in turn by weaker, and so on. This phenomenon depends upon 

 periodically recurring improvement in the nutrition of the muscle, as a result of 

 analogous variations in its circulation. 



In excised muscles also, especially if the large nerve-trunks have al- 

 ready undergone degeneration, the irritability is at first somewhat in- 

 creased after each stimulation, so that with a uniform series of stimuli 

 the contractions at first exhibit an increase in extent. Thus, it may 

 happen that, while the first weak stimulus is still ineffectual, the second 

 will give rise to a contraction. The unstriated muscles exhibit, under 

 certain conditions, automatic and rhythmic movements without the 

 intervention of nerves. 



