68 TEXT-BOOK OF PHYSIOLOGY. 



tioned hemoglobin, which gives the color to the muscles. Spectro- 

 scopic investigation reveals the presence of a special pigment, myo- 

 hematin, which is supposed to have a respiratory function, inasmuch 

 as its absorption bands change by oxidation and reduction. 



Among the extractives containing nitrogen may be mentioned 

 creatin, creatinin, xanthin, carnin, urea, uric acid, carnic acid, etc. 

 Among the extractives free of nitrogen, glycogen, dextrose, inosite, 

 lactic acid, fat, are the most important. Inorganic salts are relatively 

 abundant, of which potassium is the most abundant among the bases, 

 and phosphoric acid among the acids. 



THE PHYSICAL AND PHYSIOLOGIC PROPERTIES OF MUSCLE- 

 TISSUE. 



Consistency. The consistency of muscle-tissue during life 

 varies considerably in accordance with different states of the muscle. 

 In a state of tension it is hard and resistant ; in the absence of tension 

 it is soft and fluctuating to the sense of touch. Tension alone gives 

 rise to hardness. 



Cohesion. The cohesion of a muscle is largely dependent on the 

 quantity of connective tissue it contains. A band of fresh human 

 muscle one square centimeter in cross- section was able to resist a 

 weight of 14 kilograms without rupture (McAlister). Cohesion 

 resists the forces of traction and pressure. 



Elasticity. Muscle, in common with many other organic as 

 well as inorganic substances, is capable of being extended beyond the 

 normal length through the action of. external forces and of resuming 

 the normal length when these forces cease to act. All such bodies 

 are said to be elastic; and the greater the variations between the 

 natural and acquired lengths, the greater is their elasticity said to be. 

 Muscle therefore possesses extensibility and elasticity.* If the 

 muscle of a frog, preferably the sartorius, the fibers of which are 

 arranged in a practically parallel manner, be fastened at one ex- 

 tremity by a clamp, and then extended by a series of successive 

 weights which differ by a common increment, it will be found that 

 the extensibility of muscle does not follow the law of elasticity as 

 determined for inorganic bodies; i. e., directly proportional to the 

 weight and to the length of the body extended; but that while in- 

 creasing in length with each successive weight^! the increase is always 

 in a diminishing ratio.M Thus, for example, as shown in Fig. 19: 

 The extension produced by 5 grams is 5 millimeters, that produced 

 by 10 grams is only 4 millimeters more, and so on with additional 



* By this latter term is here meant the power by virtue of which the muscle 

 returns to its original length, and is used synonymously with perfect retractibility. 



