THE CHEMISTRY OF MUSCLE 85 



the stimulus required to cause it to contract, must always be great 

 enough to produce a contracture-like effect. Smooth muscle may also 

 be made to show the phenomenon of summation by stimulating it 

 again very soon after it has entered upon its period of relaxation. 

 This summation may be repeated until its maximal degree of short- 

 ening has been obtained which, according to Schultz, is frequently 

 73 per cent, above its resting position or abscissa. 



The character of the contraction of cardiac muscle will be discussed 

 in a later chapter dealing with the dynamic importance of the heart. 

 It may be stated at this time, however, that its contraction is inter- 

 mediate between those of striated and non-striated muscle, and is 

 most closely allied to the simple twitch of the former. Moreover, 

 cardiac muscle does not react intermittently, but possesses an auto- 

 matic power which makes it contract rhythmically in consequence of 

 the generation of certain internal stimuli. 



CHAPTER IX 



THE CHEMISTRY OF MUSCLE 



General Composition.^ — Inasmuch as the muscle tissue of an 

 adult constitutes about 42 per cent, of the body weight, it forms a 

 very considerable part of the total mass of our body. It is also very 

 important functionally, because it produces nearly 50 per cent, of the 

 total metabolism in persons at rest, and almost 75 per cent, in 

 persons undergoing moderate activity. In analyzing muscle tissue, 

 it must be taken into account that it embraces a certain amount of 

 connective tissue and also blood-vessels and nerves. Its principal 

 element is, of course, the fiber which is composed of a contractile 

 albuminous substance or sarcoplasm, and an elastin-like investnient, 

 or sarcolemma. The former possesses a semifluid or jelly-like con- 

 sistency and displays a series of doubly refracting elements. The 

 striated and non-striated types of mammalian muscle contain from 72 

 to 78 per cent, of water and from 22 to 28 per cent, of solids, the latter 

 being composed largely of proteins. 



Proteins of Muscle. — The fact that muscles become perfectly rigid after death 

 as well as on exposure to heat, has led to the belief that their albuminous constitu- 

 ents undergo a process of coagulation similar to that exhibited by the blood of the 

 warm-blooded animals. Thus, Kiihne^ has succeeded in isolating from them a 

 liquid by first freezing them and then subjecting them to a high pressure. This 

 so-called muscle-plasma clots almost immediately when slightly warmed. The 

 remaining portion of the muscle substance forms the so-called stroma. Under 

 ordinary conditions it suffices to divide the muscle into small pieces and to subject 



^ V. Fiirth, Oppenheimer's Handb. der Biochemie, Jena, 1910. 

 2 Unters. iiber das Protoplasma, Leipzig, 1864. 



