1870.] BARKER — ON VITAL AND PHYSICAL FORCES. 425 



correlation ; these shall be heat evolved within the body ; 

 muscular energy or motion ; and lastly, nervous energy, or that 

 form of force which, on the one hand, stimulates a muscle to 

 contract, and on the other, appears in forms called mental. 



The heat which is produced by the living body is obviously of 

 the same nature as heat from any other source ; it is recognized 

 by the same tests, and may be applied for the same purposes. As 

 to its origin, it is evident that since potential energy exists in the 

 food which enters the body, and is there converted into force, a 

 portion of it may become the actual energy of heat. And since, 

 too, the heat produced in the body is precisely such as would be 

 set free by the combustion of this food outside of it, it is fair to 

 assume that it thus originates. To this may be added the 

 chemical argument that while food capable of yielding heat by 

 combustion is taken into the body, its constituents are completely 

 or almost completely, oxidized before leaving it ; and since oxida- 

 tion always evolves heat, the heat of the body must have its 

 origin in the oxidation of the food. Moreover, careful measure- 

 ments have demonstrated that the amount of heat given off by the 

 body of a man weighing 180 pounds is about 2,500,000 units. 

 Accurate calculations have shown, on the other hand, that 288-4 

 gi-ams of carbon and 12-56 grams of hydrosjen are available in the 

 daily food for the production of heat. If burned out of the body, 

 these quantities of carbon and hydrogen would yield 2,765,134 

 heat units. Uurned within it, as we have just seen, 2,500,000 

 units appear as heat; the rest in other forms of energy. ^^ We 

 conceive, however, that no long argument is necessary to prove 

 that animal heat results from a conversion of energy within the 

 body ; or that the vital force heat, is as truly correlated to the 

 other forces as when it has a purely physical origin. 



The belief that the muscular force exerted bv an animal is 

 created by him is by no means confined to the very earliest ages of 

 history. Traces of it appear to the careful observer even now, 

 although, as Dr. Frankland says, science has proved that " an 

 animal can no more generate an amount of force capable of moving 

 a grain of sand than a stone can fall upward or a locomotive drive 

 a train without fuel."" In studying the characters of muscular 

 action we notice, first, that, as in the case of heat, the force which 

 it develops is in no wise different from motion in inorganic nature. 

 In the early part of the lecture, motion produced by the contrac* 

 tion of muscle, was used to show the conversion of mass-force into 



