1895.] On the Nature of Muscular Contraction. 427 



are not able to subject the isolated doubly-refractive parts of the 

 muscle in an unimpaired condition to the influence of heat. 

 Together with the elevation of temperature there occur changes in the 

 chemical processes, and therewith in the material composition and 

 mechanical properties of the whole muscle substance, which compli- 

 cate the changes dependent only on the heating of the doubly- 

 refractive particles, or even prevent our clearly recognising them. 



We may, it is true, exclude these chemical influences by previously 

 tilling the muscles by means of alcohol or some other medium, the 

 action of which preserves the finer structure of the non-contracted 

 muscle, and above all the double refraction. But in this case, too, the 

 mechanical properties of the fibrils, as well as of the sarcoplasm, 

 change so considerably that the explanation of the phenomena 

 becomes very difficult. A great part of the soft muscular substance 

 "becomes solid by coagulation or precipitation, and these solid parts 

 will act in opposition to a thermal shortening, nay, perhaps com- 

 pensate it, in consequence of their own expansion through heat. 

 Moreover, the phenomena are rendered complicated by the thermal 

 changes of other elements of the muscular tissue, such as connective 

 iibrils, blood-vessels and nerves. 



The results, however, agree very well with the hypothesis that 

 even in a dead muscle the doubly-refractive particles tend to contract 

 Tvhen heated to a certain degree, and to lengthen again when cooled. 

 The degree of heat at which quite dead, lightly loaded frogs' muscles 

 begin to shorten, does not lie much higher than those sufficient to 

 produce rigor, mostly below or near to 60 C. Contraction increases 

 "with the temperature, occasionally amounting to 20 per cent, of the 

 initial length. 



Very important contractions of the sarcous elements to the half 

 of the length and more are shown by the microscope in the striated 

 muscles of insects, when these are very rapidly heated to 55, or 

 higher. The singly-refractive layers do not contract, or at least to a 

 much less degree. However, we will not attach too much importance 

 to these facts, because the conditions are very complicated. 



Tetanus and Rigor by Heat. Living muscles, when being gradually 

 heated, are, as you know, thrown into tetanus so soon as the tempera- 

 ture has attained the height of a little below 50 C. This so-called 

 .tetanus of heat passes by prolonged heating into the lasting contrac- 

 tion of rigor, accompanied by definitive loss of irritability. 



This contraction through heat agrees in so many points with physio- 

 logical contraction, especially with physiological tetanus, that it was 

 long held to be the last manifestation of muscular life. Such points 

 of resemblance are, e.g., the amount and the force of shortening, 

 which in both cases are at least of the same order, and the increased 

 production of heat, of carbonic acid, and of a fixed acid. 



