The movements of the body do not appear to be produced in a manner similar to those of a mechanical machine, as for example, a steam engine, in which the chemical affinity of the fuel for oxygen is converted into heat; and the heat, into motion to a greater or less extent. In the first place, the energy required for the movements of the animal machine is not obtained directly from the fuel (the food), but from the machine itself (the tissues); the fuel being employed to build up and repair the component parts of this living mechanism. Secondly, the heat which is evolved does not seem capable of being utilised for the production of muscular activity; for no useful effect in that direction is obtained by attempting to prevent the dissipation of that heat into the atmosphere. It may therefore be not unreasonable to conclude that the motion for muscular work is got directly from chemical affinity, and that the evolved heat is the remaining product of that source of energy. This heat is necessary, not for muscular work, but for the performance of the vital functions, which cannot be carried on in a state of efficiency unless the system is maintained at a temperature, in the case of horses, of about 99.5° F.; the surplus heat being carried off by evaporation from the skin, lungs, etc., and by radiation and conduction. In health, when the usual supply of heat is not sufficient to maintain the body at its normal degree of temperature, the nerves stimulate the process of combustion in the system, so that more heat is produced. Hence in cold weather, animals require more food than in warm weather, and at that time, food can be more or less supplemented by clothing. The nerves also perform the duty of preventing the temperature of the body from unduly exceeding its normal standard; for when the heat production is high, as during fast work, the nerves cause an increased supply of blood to go to the surface of the body, with the result that the process of evaporation is proportionately stimulated. The regulation of the internal temperature is therefore under nervous control.