138 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



muscle-tissue of the walls of the blood-vessels ; epinephrin, 1 the active prin- 

 ciple of the extracts obtained from the medullary part of the suprarenal 

 capsules, may be mentioned here, and is of especial interest because derived 

 from the animal body. If injected into the blood, it increases the strength 

 and prolongs the contraction of the muscles generally, and causes through its 

 effect on the muscle of* the heart and the non-striated muscles of the blood- 

 vessels a marked rise of blood-pressure. 2 



Barium salts, and to a less degree calcium and strontium, act similarly 

 to veratria to prolong the relaxation of the muscle, without lessening the 

 rapidity and height of contraction. 



Potassium and ammonium salts and a large number of other chemical 

 substances and drugs act to kill the muscle, and as the death process develops 

 excitation produces prolonged localized contractions. This etfect seems to be 

 quite different from that of veratria, for it is accompanied by a rapid lessen- 

 ing of the muscular power. 



5. Liberation of Energy by the Contracting - Muscle. — The law of con- 

 servation of energy applies no less to the living body than to the inanimate 

 world in which it dwells. Every manifestation of life is the result of the 

 liberation of energy which was stored in the body in the form of chemical 

 compounds. When a muscle is excited to action it undergoes chemical 

 changes, which are accompanied by the conversion of potential into kinetic en- 

 ergy. This active energy leaves the muscle in part as thermal energy, in part 

 as mechanical energy, and, to a slight extent, under certain conditions, as elec- 

 trical energy. In general, the sum of the liberated energy is given off as heat 

 or motion. The proportion in which these two forms of energy shall be pro- 

 duced by a muscle may vary within wide limits, according to the state of the 

 muscle and the conditions under which the work is done. Fick 3 states that 

 if the muscle works against a very heavy weight, possibly one-fourth of the 

 liberated energy may be obtained as mechanical work; but if the weight be 

 light not more than one-twentieth of the chemical energy is given off in this 

 form, the muscle working no more economically than a steam engine. Zuntz 4 

 studied the work that the body as a whole could accomplish, and found that 

 somewhat more than one-third of the energy liberated can be obtained as 

 external mechanical work. The fact that always a part, and often the whole, 

 of the mechanical energy developed by the muscle is converted to thermal 

 energy within the muscle, and leaves it as heat, makes it the more difficult to 

 determine in what proportion these two forms of energy were originally pro- 

 duced. Moreover, if Engelmann's view be correct, that the change of form 

 exhibited by the muscle is the result of the imbibition of the fluid of the 

 isotropic substance by the anisotropic material, this change being brought 

 about by the heat which is liberated within the muscle, we must consider 

 potential energy to be set free first as heat, a part of which is afterward 



1 AU'l : Zeit8chrift fur phyxiologische Chemie, 1899, Bd. xxviii. S. 354. 

 '-'< Hivcr and Schafer: Journal of Physiology, 1895, xviii. pp. 230-276. 



3 Fick : Pfliiger'a Archiv, 1878, xvi. S. 85. 



4 Tbid., 1897. Bd. Ixviii., 8. 191. 



