IRRITABILITY AND STIMULATION OF THE MUSCLE. 557 



greatest excitation and the least injury. The following substances cause injury 

 in the order of their sequence, arranged from those with stronger to those with 

 weaker effects: ammonia, potassium, sodium, hydrochloric acid, nitric acid, 

 sulphuric acid, phosphoric acid (in accordance with their avidity) ; the fatty acids 

 with larger molecules as compared with those with smaller; the higher alcohols 

 as compared with the lower. 



In making experiments upon the chemical irritation of "muscles, it is inad- 

 visable to immerse the transverse section of the muscle in the solution. The 

 substance in solution should rather be applied to a limited area on the uninjured 

 surface of the muscle. The stimulation will then be manifested in a few seconds 

 by contraction or fibrillary motion of the superficial muscular layers. 



If the sartorius of a curarized frog be immersed in a solution of 5 grams of 

 sodium chlorid, 2 grams of alkaline sodium phosphate, and 0.5 gram of sodium 

 carbonate in i liter of water at 10 C., the muscle will be thrown into rhythmic 

 contractions, which may persist even for days. These contractions suggest, to a 

 certain degree, the rhythmic action of the heart (Biedermann). 



The following act as chemical irritants upon unstriated muscles: ergot, aloes, 

 colocynth, the alkalies; atropin and nicotin paralyze the nervous elements in 

 such muscles, as does also ether; chloroform also destroys the muscle-fibers them- 

 selves. Carbon dioxid in small amounts acts as an irritant to the nerves, in 

 larger amounts as a paralyzant, and in still larger amounts it irritates and finally 

 paralyzes the muscle-fibers themselves. 



3. Thermal Stimuli. If a frog's muscle be rapidly heated, a gradually in- 

 creasing contraction begins at about 28 C., becomes more pronounced at 30 C., 

 and attains its maximum at 45 C.; following this, further heating rapidly 

 leads to heat-rigor. Local cooling of the muscle increases its irritability for all 

 kinds of stimuli. Frog's muscle cooled to o is exceedingly responsive to me- 

 chanical irritation, and it may be stimulated by degrees of cold below o, until 

 freezing takes place. Heat has a relaxing effect on unstriated muscle (frog), 

 while cold has a moderately stimulating effect. Variations in temperature, how- 

 ever, also affect the nerves of these muscles, each fluctuation causing reflex con- 

 traction, which does not occur if the nerves are paralyzed. 



Cl. Bernard made the remarkable observation that the muscles of artificially 

 cooled animals retain their irritability for many hours after death. Heat causes 

 rapid disappearance, with temporary increase of the irritability. 



4. Mechanical stimuli of all kinds cause a contraction at each separate, sudden 

 blow; and tetanus if repeated. Strong, local stimuli induce an elevated contrac- 

 tion of considerable duration at the point of application. Moderate stretching of 

 a muscle increases its irritability. Mechanical stimulation of a muscle poisoned 

 with veratrin causes a heaving movement of its fibers, which may persist for as 

 long as one minute. 



5. Electrical stimuli are discussed in conjunction with nerve-stimuli (p. 631). 

 Curare, the arrow-poison of the South American Indians, is the dried juice 



of the root of Strychnos Crevauxi. When introduced into the blood or injected 

 subcutaneously, it first causes paralysis of the intramuscular termination of the 

 motor nerves, the muscles themselves retaining their irritability, while the sensory 

 nerves and those of the central organs and the viscera (heart, intestine, and ves- 

 sels) remain for a time unaffected. In warm-blooded animals the paralysis of 

 the respiratory muscles naturally causes early asphyxia, which is unattended with 

 convulsions. Frogs, whose skin is their most important respiratory organ, on 

 receiving a suitable dose, may recover completely, after remaining motionless for 

 days, during which the poison is eliminated through the urine. Larger doses paralyze 

 also the cardiac inhibitory and vasomotor nerves. In electrical fish paralysis of 

 the nerve transmitting the electrical shock occurs. In frogs the lymph-hearts also 

 are paralyzed. If the doses that are fatal when administered subcutaneously be 

 given by mouth, poisoning does not result, because the poison is eliminated by 

 the kidneys at the same rate that it is absorbed by the gastric mucous membrane. 

 For the same reason the flesh of an animal killed by a poisoned arrow is harm- 

 less. If, however, the ureters be ligated, the poison accumulates in the blood, 

 and intoxication results. Large doses, however, will kill uninjured animals also 

 by way of the intestinal tract. 



Atropin appears to be a specific poison for unstriated muscle-fibers, although 

 different muscles are variously affected by it. 



The irritability of the muscles after lesions of the nerves deserves especial 

 attention. After three or four days the irritability of the paralyzed muscle is 

 diminished for direct or indirect (nerve) stimuli. There then follows a stage in 



