13. It may be concluded then, that respiration, circulation, and nervous influence, all co operate in producing animal heat, or that they are conditions essential to this phenomenon. We here also perceive a circle of actions ; or a chain, composed of many links, no one of which can be spared. Each one contributes to a result essential to organic existence, and one of the most curious and wonderful which the mind can contemplate.

14. If these principles be correct, we should expect to find the greatest heat in those animals whose structure is the most complex; whose organic or vital actions are the most intense; and whose changes are the most rapid. Now all these conditions belong to the warm blooded animals especially, and particularly to birds, whose temperature exceeds, by several degrees, that of man. In them, as I have already stated, respiration is more perfect, and the circulation more rapid.

15. Upon the same principles, it would also follow, that animal heat would be the highest in those portions of the body where the organic actions are most active. And such is actually the case. If a thermometer be placed over the chest, the stomach, or the liver, it will indicate a higher temperature by some degrees than exists in the extremities, because it is near the seat of respiration, digestion, and the largest secretory organ in the body. The temperature of different parts of the animal body ranges from 91 to 100 degrees ; and in fever to 112, which is marked "fever heat" on the thermometer.

16. Those animals that belong to the class mammalia, and which live in water, such as the whale, the dolphin, and the porpoise, have a temperature as high as that of man. Captain Scoresby found the temperature of a whale, in the Arctic Ocean, to be 104 degrees; while that of the polar bear and wolf were no greater. Warm blooded animals are generally covered with hair or feathers, to protect them from the cold; cold blooded animals need no protection, as they can live in a temperature as low as the freezing point of water. Seals, bears and walruses in the Northern Ocean, are protected by a coating of hair; the whale is protected by the great thickness of its skin and the stratum of fat im mediately beneath it. Dr. Edwards states that frogs, which can live in water at 32 degrees, will die in a short time in water at 105 degrees.

17. But this does not seem to be the case in all coldblooded animals. In some of the warm springs in Brazil, which have a temperature of 88, many small fishes are found; and also in a hot spring at the Manillas, which raises the thermometer to 158 degrees; and Humboldt, in his travels, tells us that in the province of Quito, in South America, he saw fishes thrown up from the bottom of a volcano, along with water and heated vapour, which raised the thermometer to 210, or only two degrees short of the boiling point. Lord Bute saw beetles in the boiling springs of AI-bano, which died when plunged into cold water; and Dr. Elliotson states that a gentleman of his acquaintance boiled some honeycomb two years old, and after extracting all the honey, threw the refuse into a stable, which was soon filled with bees.

18. Warm blooded animals have the faculty of preserving the same degree of heat in nearly every variety of climate. During Captain Parry's voyage to the Arctic seas, in quest of the northwest passage, the crews of his vessels were often exposed to a temperature of fifty degrees below zero, or one hundred and fifty degrees below that of their own bodies, and still they were able to resist it, and escape being frost bitten. When the temperature was thirty two degrees below zero, they found that of an Arctic fox one hundred and six degrees, which shows what a strong counteracting energy there is in animals, against the effects of cold.

19. The human body can also resist great heat as well as cold. In summer we are often exposed, when in the sun, to a temperature many degrees above that of the body, but the heat of the body is not elevated. Chantry, the sculptor, often entered his furnace when heated for drying his moulds, when the thermometer in it stood at three hundred and twenty; and his workmen did the same when the temperature was three hundred and forty degrees. Dunglison states that Chabert entered an oven with impunity, the heat of which was from four hundred to six hundred degrees. In all these cases, where even water was boiled, and meat cooked to a crisp, the heat of the body was raised but a few degrees.

20. In these cases the heat of the body is kept down to near the natural standard, by exhalation, or sweating. This carries off the heat in a state of vapour as fast as it is produced. For the same reason, water cannot be heated above two hundred and twelve degrees, as it then escapes in the form of vapour. By evaporation, also, bottles of wine are cooled in summer, by wrapping them round with wet cloths. In India, it is said, that ice is produced in a similar manner. If an animal be saturated with moisture, and placed in a hot oven, it soon dies, because the exhalation from the body is prevented. It is for this reason, also, that we feel the heat more in damp weather than in dry, although the temperature be lower. Such weather is called sultry, close, muggy, etc, because the saturation of the air by moisture, prevents the escape of heat by evaporation from our bodies.

21. In infancy, the faculty of generating animal heat is much less than in adult age. This is the chief cause why so many perish during the first years of Hfe, and particularly in winter, during the first few days. At this period, the temperature of infants is only about ninety three degrees, and in some instances not more than eighty, and is rapidly diminished by slight exposures. This shows the great importance of guarding against the effect of cold, by proper clothing, and a regulated temperature of houses. Man has no natural protection against the influence of cold, but reason was given him as a substitute. How few appear to employ this faculty, in guarding against atmospheric vicissitudes by a proper adaptation of the dress; and in consequence, how many, instead of consulting their health, comfort, and usefulness, fall victims to negligence or fashion !

Questions

What is calorification ? What are the causes of animal heat ? Why was it surmised that respiration was concerned in this process ? What objections to this ? What was Crawford's hypothesis ? What occurs when respiration is impeded ? What occurs in coldblooded animals ? How much heat, according to Magendie, is produced by respiration in herbiverous animals ?-in carniverous ?-in birds ? Is arterial blood warmer than venous ? How much heat does the blood acquire in passing through the lungs ? How many degrees does the system acquire from this source in 24 hours ? What is Brodie's theory with respect to animal heat ? What facts in favour of it ? What is said of animal heat being generated in the capillary system ? What axiom in relation to a change in the particles of bodies has a bearing on this subject ? What conditions are necessary to the production of animal heat by the action of the capillaries? How is this proved?

What functions co operate in the production of animal heat ? In what animals is heat the greatest ? Why ? In what parts of the body is it the highest ? Why ? What is the temperature of the mammalia that live in water? What effect does a change of temperature have on cold blooded animals ? Can warm blooded animals preserve the same temperature in every climate ? How is this effected ? What effect will checking evaporation have ? Is the faculty of generating heat less in infancy ? What inference do we draw from this fact ?