acid varies according to the age; and that independently of the weight of the individual subjected to experiment. 3. During all the periods of life, from that of eight years up to the most advanced age, the male and female may be distinguished by the different quantities of carbonic acid which they exhale in a given time. Other things being equal, the male exhales always a larger quantity than the female. This difference is particularly marked between the ages of 16 and 40 years, during which period the male usually exhales twice as much carbonic acid as the female. 4. In the male, the quantity of carbonic acid increases constantly from eight to thirty years; and the rate of this increase undergoes a rapid augmentation at the period of puberty. Beyond thirty years the exhalation of carbonic acid begins to decrease, and its diminution is more marked as the individual approaches extreme old age; so that near the termination of life, the quantity of carbonic acid produced may be no greater than at the age of ten years. 5. In the female, the exhalation of carbonic acid increases according to the same law as in the male, from the age of eight years until puberty. But at the period of puberty, at the same time with the appearance of menstruation, the exhalation of carbonic acid, contrary to what happens in the male, ceases to increase; and it afterward remains stationary so long as the menstrual periods recur with regularity. At the cessation of the menses, the quantity of carbonic acid exhaled increases in a notable manner; then it decreases again, as in the male, as the woman advances toward old age. 6. During the whole period of pregnancy, the exhalation of carbonic acid rises, for the time, to the same standard as in women whose menses have ceased. 7. In both sexes, and at all ages, the quantity of carbonic acid is greater as the constitution is stronger, and the muscular system more fully developed. Prof. Scharling, in a similar series of investigations,' found that the quantity of carbonic acid exhaled was greater during the digestion of food than in the fasting condition. It is greater, also, in the waking state than during sleep; and in a state of activity than in one of quietude. It is diminished, also, by fatigue, and by most conditions which interfere with perfect health. The process of respiration is not altogether confined to the lungs, Annales de Chimie et de Pharmacie, vol. viii. p. 490. but the interchange of gases takes place, also, to some extent through the skin. It has been found, by inclosing one of the limbs in an air-tight case, that the air in which it is confined loses oxygen and gains in carbonic acid. By an experiment of this sort, performed by Prof. Scharling,' it was ascertained that the carbonic acid given off from the whole cutaneous surface, in the human subject, is from onesixtieth to one-thirtieth of that discharged during the same period from the lungs. In the true amphibious animals, that is, those which breathe by lungs, and can yet remain under water for an indefinite period without injury (as frogs and salamanders), the respiratory function of the skin is very active. In these animals, the integument is very vascular, moist, and flexible; and is covered, not with dry cuticle, but with a very thin and delicate layer of epithelium. It, therefore, presents all the conditions necessary for the accomplishment of respiration; and while the animal remains beneath the surface, and the lungs are in a state of inactivity, the exhalation and absorption of gases continue to take place through the skin, and the process of respiration goes on in a nearly uninterrupted manner. In Carpenter's Human Physiology, Philada. ed., 1855, p. 308. CHAPTER XIII. ANIMAL HEAT. ONE of the most important phenomena presented by animals and vegetables is the property which they possess of maintaining, more or less constantly, a standard temperature, notwithstanding the external vicissitudes of heat and cold to which they may be subjected. If a bar of iron, or a jar of water, be heated up to 100° or 200° F., and then exposed to the air at 50° or 60°, it will immediately begin to lose heat by radiation and conduction; and this loss of heat will steadily continue, until, after a certain time, the temperature of the heated body has become reduced to that of the surrounding atmosphere. It then remains stationary at this point, unless the temperature of the atmosphere should happen to rise or fall; in which case, a similar change takes place in the inorganic body, its temperature remaining constant, or varying with that of the surrounding medium. With living animals, the case is different. If a thermometer be introduced into the stomach of a dog, or placed under the tongue of the human subject, it will indicate a temperature of 100° F., very nearly, whatever may be the condition of the surrounding atmosphere at the time. This internal temperature is the same in summer and in winter. If the individual upon whom the experiment has been tried be afterward exposed to a cold of zero, or even of 20° or 30° below zero, the thermometer introduced into the interior of the body will still stand at 100° F. As the body, during the whole period of its exposure, must have been losing heat by radiation and conduction, like any inorganic mass, and has, notwithstanding, maintained a constant temperature, it is plain that a certain amount of heat has been generated in the interior of the body by means of the vital processes, sufficient to compensate for the external loss. The internal heat, so produced, is known by the name of vital or animal heat. There are two classes of animals in which the production of vital heat takes place with such activity that their blood and internal organs are nearly always very much above the external temperature; and which are therefore called "warm-blooded animals." These are mammalia and birds. Among the birds, some species, as the gull, have a temperature as low as 100° F.; but in most of them it is higher, sometimes reaching as high as 110° or 111°. In the mammalians, to which class man belongs, the animal temperature is never far from 100°. In the seal and the Greenland whale, it has been found to be 104°; and in the porpoise, which is an air-breathing animal, 99.5. In the human subject it is 98° to 100. When the temperature of the air is below this, the external parts of the body, being most exposed to the cooling influences of radiation and conduction, fall a little below the standard, and may indicate a temperature of 97°, or even several degrees below this point. Thus, on a very cold day, the thinner and more exposed parts, such as the nose, the ears, and the ends of the fingers, may become cooled down considerably below the standard temperature, and may even be congealed, if the cold be severe; but the temperature of the internal organs and of the blood still remains the same under all ordinary exposures. If the cold be so intense and long continued as to affect the general temperature of the blood, it at once becomes fatal. It has been found that although a warm-blooded animal usually preserves its natural temperature when exposed to external cold, yet if the actual temperature of the blood become reduced by any means more than 5° or 6° below its natural standard, death inevitably results. The animal, under these circumstances, gradually becomes torpid and insensible, and all the vital operations finally cease. Birds, accordingly, whose natural temperature is about 110°, die if the blood be cooled down to 100°, which is the natural temperature of the mammalia; and the mammalians die if their blood be cooled down below 94° or 95°. Each of these different classes has therefore a natural temperature, at which the blood must be maintained in order to sustain life; and even the different species of animals, belonging to the same class, have each a specific temperature which is characteristic of them, and which cannot be raised or lowered, to any considerable extent, without producing death. While in the birds and mammalians, however, the internal production of heat is so active, that their temperature is nearly always considerably above that of the surrounding media, and suffers but little variation; in reptiles and fish, on the other hand, its produc tion is much less rapid, and the temperature of their bodies differs but little from that of the air or water which they inhabit. Birds and mammalians are therefore called "warm-blooded," and reptiles and fish "cold-blooded" animals. There is, however, no other distinction between them, in this respect, than one of degree. In reptiles and fish there is also an internal source of heat; only this is not so active as in the other classes. Even in these animals a difference is usually found to exist between the temperature of their bodies and that of the surrounding media. John Hunter, Sir Humphrey Davy, Czermak, and others,' have found the temperature of Proteus anguinus to be 63°.5, when that of the air was 55°.4; that of a frog 48°, in water at 44°.4; that of a serpent 88°.46, in air at 81°.5; that of a tortoise 84°, in air at 79°.5; and that of fish to be from 1°.7 to 20.5 above that of the surrounding water. The following list shows the mean temperature belonging to animals of different classes and species. In the invertebrate animals, as a general rule, the internal heat is produced in too small quantity to be readily estimated. In some of the more active kinds, however, such as insects and arachnida, it is occasionally generated with such activity that it may be appreciated by the thermometer. Thus, the temperature of the butterfly, when in a state of excitement, is from 5° to 9° above 'Simon's Chemistry of Man, Philadelphia edition, p. 124. |