912 SPECIAL PHYSIOLOGY. 



sential to muscular and nervous action, to the development of animal 

 heat, to the electrical phenomena of living bodies, and to the evolution 

 of light in animals. So, too, certain mechanical work, performed ex- 

 clusively within the animal body, must, when completed, pass into 

 heat, as the result of arrest, concussion, or friction. Heat, again, is 

 necessary for the solvent processes accompanying the digestion and 

 absorption of the food; and it exercises a well-known influence upon, 

 and often determines, the quantity of the chemical change and dy- 

 namic work performed in the body. 



In the Inorganic world considered exclusively, gravitation and solar 

 heat are the chief modes in which force is manifested. The evapora- 

 tion of water from the surface of the earth, its conversion into clouds, 

 its descent in the form of fogs, rain, snow, or hail, the formation of 

 glaciers, mountain-streams, and rivers; and the production of ascend- 

 ing, descending, and horizontal currents in the atmosphere, are the 

 evidences of these forms of energy. Oxidation and other chemical 

 changes, though not absent, are comparatively inactive in the present 

 condition of the inorganic world. 



In the Organic world, however, in plants and animals, chemical 

 change constitutes the most essential modes or forms of force, and the 

 source of the other forms of force manifested by them. Under the 

 influence of certain of the solar rays, differing from the simply heating 

 rays, viz., the luminous and the actinic rays, the deoxidation and fixa- 

 tion of certain elements take place in plants ; and in these elements 

 so fixed and combined, a force, derived from the solar rays, is then 

 stored up. In animals, again, oxidation is the essential phenomenon, 

 an opposite chemical change occurs, the force stored up in the animal 

 blood or tissues, which is but a transfer of that of the vegetable con- 

 stituents of the food, is, together with the force proper to the oxygen 

 of the air, then liberated, and, by the special organic apparatus of the 

 animal body, is changed, as required, into other modes of action, mus- 

 cular, nervous, thermic, digestive, or excretory, necessary for the 

 maintenance of animal existence. In supplementing the mechanical 

 forces of nature dependent on gravitation or solar heat, such as wind- 

 and water-power, Man has had recourse to chemical change, as a source 

 for the production of heat and mechanical force. The carbon and 

 hydrogen of coal are made to unite with oxygen ; from this combina- 

 tion, heat is evolved; by this, water is converted into steam ; and, by 

 the expansive force of the latter, the requisite motion is obtained. An 

 obvious comparison is here suggested between a machine and the body, 

 between the force obtained by the combustion of dead matter and the 

 oxidation of the living tissues; and, lastly, between the working of a 

 steam engine and the muscular movements. 



In general physics, results, to be of scientific value, must be ex- 

 pressed numerically. The quantity of fuel and oxygen undergoing 

 change in combustion, is accurately determined by weight or volume; 

 the relative amount of heat evolved, is ascertained and recorded; and 

 if the heat be applied, as by expansion for mechanical purposes, the 

 value of the work it performs is exactly measured. By such means, 

 the amount of each kind of force manifested, is expressed in numbers, 



