ANIMAL IIEAT 523 congealed and destroyed, without affecting sensibly the general system. But if the cold be so intense and long continued as to depress the general temperature of the blood and the internal organs, the system at large begins to feel its effects, and the vital powers yield to its influence. A benumbing effect is produced, followed by a difficulty of muscular exertion, a confusion of mind, drowsiness, and insensi- bility ; and death takes place long before the body as a whole is actually congealed. Thus the maintenance of the internal temperature at or near the natural standard is a condition necessary to life. Experiments upon the warm- blooded animals have shown that in them, as a general rule, death is produced when the tem- perature of the blood is reduced to about 80. The vital changes necessary to existence can- not go on below this point. On the other hand, the animal temperature may rise above the natural standard. There is no doubt that an increase of heat is produced in the muscular tissue during the contraction of these organs. We have already noticed the rise of tempera- ture observed by Mr. Newport in a hive of bees when the insects were excited to activity. Becquerel and Breschet found the temperature of the biceps muscle of a man raised 1'8 by active contraction and relaxation continued for several minutes ; and Matteucci observed an increase of 1 in the muscle of a frog separated from the body and artificially excited to con- traction. It is a matter of common observation that a general sensation of unusual warmth follows any active muscular exertion. Not only is the temperature of the muscular system itself raised, but the rapidity of the circulation is accelerated, a larger quantity of warm blood is brought to the skin in a given time, and the sensitive integument thus feels the increased temperature. No doubt it is owing to this fact that active muscular exercise is itself a protec- tion against external cold. An unusual degree of heat in the atmosphere also tends indirectly to raise the temperature of the body; for if the internal production of heat be the same, and its external loss by contact with the at- mosphere be diminished, of course the actual temperature of the body would rise in conse- quence. A provision is made, however, against allowing this increase of temperature, whether from muscular exertion or external heat, to reach too high a point. This provision is the cutaneous perspiration. Anything which raises the bodily heat above the natural stand- ard excites the circulation through the skin, and increases the quantity of perpsiration pour- ed out upon its surface. This fluid, by its evap- oration, uses up or renders latent a portion of the heat, and thus reduces the skin and the blood circulating through it to its natural tem- perature. The body therefore can be exposed to a very high external temperature without itself rising above its natural standard, provided the perspiration be free and its evaporation unim- peded. If the perspiration be checked, how- ever, or if its evaporation be prevented by ex- posure to hot water, or hot air loaded with moisture, the" temperature of the body rises, and death soon takes place. The experiments of Magendie and others have shown that in the higher animals life is destroyed when the blood generally has become heated 10 or 13 above the natural standard. Animals therefore have a natural internal temperature, which is essen- tial to the performance of the vital functions, and which cannot be either raised or lowered to any considerable extent without producing death. With regard to the precise mode in which animal heat is generated, and its exact chemical conditions, opinions are not entirely agreed. Many physiologists have entertained and still accept the belief that it is due to an oxidation or combustion of the elements of the blood and tissues by the oxygen absorbed in respiration. The grounds for this doctrine are as follows: 1. The most common and ready method by which heat is generated artificially is the combustion of substances, like wood and coal, which are rich in carbon. The rapid oxi- dation of these substances, which requires a free access of air, causes a great development of heat, and at the same time uses up the oxy- gen of the atmosphere, and produces as a re- sult carbonic acid. The consumption of fuel, the degree of heat produced, and the quantities of oxygen absorbed and carbonic acid liberated, are all in direct ratio to each other. The pro- cess may go on rapidly or slowly ; but in either case the relations of quantity remain the same. If the oxidation be rapid, as in a furnace or open fireplace with a strong draught, the fuel is soon consumed and a large quantity of heat is pro- duced in a given time. If the process be re- tarded, as in a close stove with a limited or gradual admission of air, the consumption of fuel is slow, and the heat, less intense at any particular moment, is continued for a propor- tionally longer time. But in both instances, for the entire amount of heat which has been gen- erated, there are the same quantities of fuel consumed, of oxygen absorbed, and of carbonic acid produced. 2. In the animal body the ab- sorption of oxygen and the exhalation of car- bonic acid are the most striking and constant of all the phenomena of nutrition. At the same time heat is evolved, as in the case of artificial combustion ; and it is very natural to connect the two sets of phenomena with each other. Furthermore, as in artificial combustion, the elevation of temperature in different animals, corresponds very closely with the activity of respiration and the quantity of the two gases inspired and exhaled. These considerations have led to the adoption of the theory, at once intelligible and comprehensive, which attributes the production of animal heat to the direct oxi- dation or combustion of the carbonaceous in- gredients of the food and tissues. On the other hand, there are certain facts which are less favorable to the above theory. 1. In the first place, though the combustion of carbonaceous
