Ammal Heat, the heat produced in the interior of animal bodies by the nutritive changes going on in the blood and the tissues. Living animals, as a general rule, if not invariably, have the power of generating heat within their own bodies. The proof of this is, that in many of them the temperature of the body is habitually above that of the surrounding atmosphere or water in which they live. Thus the temperature of the porpoise has been found to be 99.5° F., and that of the seal 104°. The temperature of the human body, and that of the quadrupeds generally, is about 100°; while that of many of the birds is 105°, 110°, or even 111°. As this temperature is maintained at or about the same standard, though that of the external atmosphere may be much lower, and as the animal is consequently losing heat incessantly by radiation and conduction, it is evident that there is a constant supply from an internal source by which the external loss is made good. In man and in all the higher animals, namely, birds and mammals, this internal heat is very active; so much so that their higher temperature is easily distinguished both by the touch and the thermometer, and is kept at almost a uniform standard whatever may be the external variations.

These are therefore called the warm-blooded animals. In reptiles and fishes, on the other hand, the production of heat is less active; their temperature is habitually lower than our own, so that they feel cool to the touch; and it varies so little from that of the surrounding media that greater care is requisite to distinguish it, even by the thermometer. They are accordingly distinguished as the cold-blooded animals. Animal heat is generated, however, even in these species, as is demonstrated by exact observation. Thus the temperature of a frog has been found to be 48° when immersed in water at 44.4°; that of a serpent 88,4°, in air at 81-5°; that of a tortoise 84°, in air at 79,5°; and that of a fish from 1.7° to 2.5° above that of the surrounding water. In the invertebrate animals, the appreciation of their temperature by the thermometer has been found more difficult, since, on account of their small size, the radiating external surface is greater in proportion to the mass of heat-producing tissue within; and the heat thus generated is almost as rapidly dissipated. This difficulty, however, has been overcome in the case of insects by experimenting upon a large number collected in a small space.

Thus Mr. Newport found that when the temperature of the external atmosphere was 34.5°, that of the interior of a hive of bees was 48.5°; and that if the insects were thrown into a state of active excitement by rapping on the hive, it would rise to 102°. - The heat thus produced in the interior of the body is not exactly the same in degree in every part. It is generated either in the blood itself or in the substance of the internal organs, or most probably in both. At all events, the blood acquires during its circulation through different organs slightly different degrees of warmth. Thus Claude Bernard has found, by introducing the bulb of a delicate thermometer into the vessels of a living dog, that the temperature of the blood in the abdominal aorta varied from 99.5° to 105.5°; in the portal vein, from 100° to 106°; and in the hepatic vein, from 101° to 100.8°. The warmest blood in the body, on the average, was that of the hepatic vein, which had passed through two successive capillary circulations, namely, that of the intestines and that of the liver, since leaving the arterial system. On the other hand, while passing through an organ in which it is exposed to the influence of air and evaporation, the blood diminishes somewhat in temperature.

Thus, in passing through the lungs it was found to have lost sometimes a little less and sometimes a little more than 1/3° F. For the same reason, the temperature of the skin is habitually a little lower than that of the internal organs. If the bulb of a thermometer be taken between the lingers of the closed hand, it will rise only to 90° or 95°; in the axilla, carefully protected from the air, it will stand at 98°; while under the tongue, and in contact only with the vascular mucous membrane, it will reach 100°. In the external parts of the body, therefore, which are especially exposed to the influence of the outer air, the temperature may vary considerably. Especially the thinner parts, with a comparatively greater extent of surface, feel this variation in a marked degree, and may thus be affected with a local depression of temperature. On a very cold day the ends of the fingers, the nose, the ears, etc, may be cooled down very considerably, and in some instances may be even 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 insensibility; 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 warmblooded animals have shown that in them, as a general rule, death is produced when the temperature of the blood is reduced to about 80o. The vital changes necessary to existence cannot 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 temperature 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 1.8o 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 contraction. It is a matter of common observation that a general sensation of unusual warmth follows any active muscular exertion.