n CHANCE OF FORM IN MUSCLE DURING ACTIVITY 103 



" tonus." External stimuli are for the most part the immediate 

 cause of the latter, although a certain degree of tonus seems to 

 l>e present under normal conditions without additional stimuli. 

 We have frequently observed a persistent uniform state of con- 

 traction in the ventricle of the snail's heart (Helix pom<iti), 

 after a greater or lesser series of regular contractions conse- 

 quent on sudden increase of pressure (Biedermann, 36). This 

 condition is invariably developed in the same way in a heart 

 attached to a canula and filled with snail's blood or 0'5 



/ 



salt solution. The ventricle at first extends itself ad maxi- 

 iii t'ni under the total pressure of the column of fluid in the 

 canula, and empties again completely at each systolic contrac- 

 tion, but it is soon evident that the relaxation at diastole is 

 incomplete. There is, so to speak, a contraction residue which 

 grows with each successive contraction, until finally the heart 

 ceases to relax, and remains in permanent (tonic) systolic con- 

 traction. The tonus may be resolved under certain conditions if 

 the preparation is exposed to a higher temperature, while it 

 reappears on cooling. This " cold tonus " apparently reduces 

 much more rapidly on heating than the " pressure tonus." A 

 single, momentary immersion in warm salt solution usually suffices 

 to bring the contracted ventricle, with a scarcely perceptible 

 " latent period," into the condition of complete diastolic relaxation. 



A question which naturally belongs here, relates to the upper 

 and lower limits of temperature at which a muscle is, generally 

 speaking, capable of functioning, or at any rate can recover its 

 capacity to function. 



There is nothing surprising in the fact that muscle, like 

 protoplasm in general, may be cooled to below C., without 

 permanent loss of excitability, for the freezing-point of the inter- 

 stitial tissue-fluids, as well as that of contractile substance, must 

 necessarily lie below zero. But it is difficult to say in detail 

 what kind of changes the muscle-substance undergoes when its 

 capacity of reaction is almost abolished by cooling. At all events 

 the intensity of metabolism is reduced to a minimum. According 

 to Gad and Heymans, restoration is impossible when reaction 

 ceases entirely, upon which the excitable substance must have 

 been injured intrinsically. This may occur either from its actual 

 freezing, or from mechanical injury due to the freezing of the 

 interstitial tissue-fluids. Kiihne and Hermann, and more recently 



