742 THE PHYSIOLOGY OF 7 HE CONTRACTILE TISSUES 



Of the nitrogenous extractives, kreatin (p. 587) and carnosin are 

 present in greatest quantity, muscle containing 0-2 to 0-4 per cent, of 

 kreatin. Carnosin (C 9 H 14 N 4 O 3 ) is a substance with basic properties, 

 and can be split up into histidin and /3-amino-propionic acid, an 

 amino-acid not identical with alanin (or a-amino-propionic acid), but 

 having the NH 2 coupled to the jrf instead of the a carbon atom (p. 557). 



There is more water in the muscles of young than of old animals 

 (v. Bibra), and more in tetanized than in rested muscle (Ranke). The 

 fats are variable in amount, and belong to a small extent to the actual 

 muscle-fibres. For even when the visible fat is separated with the 

 utmost care, nearly i per cent, of fat still remains (Steil). 



The glycogen content varies extremely in different muscles and in 

 the same muscle under different nutritive and functional conditions. 

 Thus, in one and the same dog the biceps brachii contained 0*17 and 

 the quadriceps femoris 0*53 per cent. In dogs on a diet rich in carbo- 

 hydrate and protein the percentage in the whole skeletal musculature 

 ranged from 0-7 to 3*7, and in the heart from o-i to 1-2. The average 

 for human muscles has been given as 0-4 per cent. In lean horse-flesh 

 Pfliiger found 0-35 per cent, of glycogen, but no sugar. The total 

 nitrogen was 3*21 per cent, of the moist tissue. The lactic acid of 

 muscle and other tissues is the d-lactic acid, which rotates the plane of 

 polarization to the right. By the action of certain bacteria on cane- 

 sugar /-lactic acid is obtained, which is left rotatory. The optically 

 inactive fermentation lactic acid is obtained by the fermentation of 

 lactose. 



Smooth muscle is somewhat richer in water than the striated variety 

 from the same species, because skeletal muscle is richer in fat. Glycogen 

 is either absent or present only in traces in the smooth muscle (of the 

 stomach and bladder). Lactic acid, kreatin, and kreatinin are also 

 found in much smaller amount than in striped muscle (Mendel and Saiki) . 

 As in striated muscle, hypoxanthin is the conspicuous purin base 

 occurring in the free form i.e., obtainable in muscle extracts. The 

 most remarkable difference in the quantitative relations of the inorganic 

 constituents is that in striated muscle potassium preponderates over 

 sodium and magnesium over calcium, whereas in the smooth variety 

 this relation is reversed. 



It would be natural to expect that the proteins, which bulk so 

 largely among the solids of the dead muscle, and which are so obvi- 

 ously important in the living muscle, should be affected by contrac- 

 tion. But up to the present time no quantitative difference in the 

 proteins of resting and exhausted muscle has ever been made out. 

 The quantity of kreatin (and kreatinin) is said by some authorities 

 to be increased. The following chemical changes have been defi- 

 nitely established. In an active muscle 



(a) More carbon dioxide is produced, (b) More oxygen is consumed. 

 (c) Lactic acid is formed, (d) Glycogen is used up. (e) The substances 

 soluble in water diminish in amount; those soluble in alcohol increase. 



Production of Carbon Dioxide and Consumption of Oxygen during 

 Contraction. This subject has already been dealt with in part in 

 connection with tissue respiration (p. 265). The fact that muscular 

 exercise increases the carbon dioxide output and the oxygen absorp- 

 tion at the pulmonary surface, shows that oxidation processes 

 involving ultimately the combustion of carbon-containing substances 



