550 THE SOURCE OF MUSCULAR ENERGY. 



retain their irritability longer in oxygen or in air than in indifferent 

 gases free from oxygen, it is to be assumed that this gaseous interchange 

 is a vital phenomenon connected with normal metabolism, and to which 

 the functional activity of the muscle is due. 



The excised, resting, surviving muscle gives off carbon dioxid, which in part 

 has been present in the muscle preformed, and in part is subsequently generated 

 by processes of decomposition that accompany the development of rigidity. A 

 small part of this carbon dioxid arises only when oxygen is supplied. Bacterial 

 putrefaction of the muscles causes marked excretion of carbon dioxid. 



In active muscle the blood-vessels are always dilated, and the amount 

 of blood passing through them is increased three or four times, a cir- 

 cumstance that obviously indicates increased metabolic activity. Ac- 

 cordingly, active is distinguished from passive muscle by a series of 

 chemical changes: 



1. The contents of living passive muscle have an alkaline, or, more 

 correctly, a neutral reaction, changing red litmus to blue, but acid to 

 turmeric paper. The reaction becomes acid in active muscle (not of 

 the unstriated variety), and, indeed, the degree of acidity increases, to 

 a certain limit, in proportion to the amount of work performed. The 

 acidity is due to phosphoric acid resulting from the decomposition of 

 lecithin and nuclein. 



The earlier view, that the acidity is due to the development of lactic acid 

 produced from glycogen, has not been substantiated. Pfliiger and Warren, and 

 also Astaschewsky and Heffter, even found the quantity of lactic acid in active 

 muscles diminished, as compared with passive muscles. Other investigators, how- 

 ever, still adhere to the theory of lactic-acid formation, especially if there is a 

 deficiency of oxygen during the work. 



2. The active muscle excretes considerably more carbon dioxid than 

 the resting muscle: (a) Active muscular exertion in man or animals 

 increases considerably the excretion of carbon dioxid from the body, 

 (b) Venous blood flowing from the tetanized muscles of an extremity 

 contains an increased quantity of carbon dioxid; and, indeed, under 

 these conditions more carbon dioxid is excreted than corresponds to the 

 amount of oxygen simultaneously absorbed, (c) Also, excised, con- 

 tracted muscles excrete an increased amount of carbon dioxid. 



3. Active muscle consumes a greater amount of oxygen: (a) During 

 work the entire body takes up much more oxygen, even four or five 

 times as much, (b) Venous blood flowing from the active muscles of 

 an extremity contains a diminished amount of oxygen. Nevertheless, 

 the increase in the consumption of oxygen by an active muscle is not so 

 great as the increase in the excretion of carbon dioxid. The increase in 

 the interchange of gases continues in the period of rest immediately 

 following the activity. 



The consumption of oxygen can also be demonstrated volumetrically 

 in excised muscles deprived of blood. It is true, oxygen is not absolutely 

 necessary for muscular activity of short duration, as the excised muscle 

 is capable of contracting for some time in a vacuum or in a gaseous mixture 

 free from oxygen, and no free oxygen can be obtained from its tissue. 

 The muscle must, therefore, contain a supply of oxygen in chemical 

 combination, which is consumed during activity. Frogs' muscles ab- 

 stract the oxygen from easily reducible substances; thus, they may de- 

 colorize a solution of indigo. Muscles that have rested act less energet- 

 ically than those that have been active. 



