Some idea of the activity of the blood flow which equalizes the body temperature may be obtained from the observations of Burton-Opitz,1 from which may be calculated that an amount of blood equal to the entire amount in the body of a dog traverses the liver every three minutes.

It has been seen that Lavoisier noticed that cold increases the metabolism. This has been abundantly confirmed. The simplest illustration of this action is to be found in fasting animals. Rubner has called this increase of metabolism and, therefore, of heat production the chemical regulation of the body temperature (see p. 118). It is the same as burning more coal in the furnace on a cold day in order to maintain the temperature of the house. Voit had previously demonstrated this action in the case of a man (see below).

It has been noted that a constant basic quantity of energy is necessary to maintain the life-processes of a warm-blooded animal situated in a tropical environment. In this case the energy of metabolism is directly concerned in maintaining the vibrant motions of the molecules of protoplasm (see p. 301) and heat production is a secondary result. If, now, the organism be subjected to the influence of a cold environment, there is an increased production of heat which is directly derived from metabolized substances and the mission of which is to maintain the temperature of the body at the tropical point. It will also be shown in another place how this passive increase in heat production through "chemical regulation," which is induced without visible motion on the part of the animal, may become unnecessary if instead the needed heat be obtained from other sources, as from the increased heat production incident to muscular work or to food ingestion.

Rubner placed a fasting guinea-pig in a bell-jar which was ventilated so that the carbon dioxids production could be determined. The temperature of the bell-jar could be changed by immersing it in water. The following were the results:

1 Burton-Opitz: "Quarterly Journal of Experimental Physiology," 1912, v, 189.

Action Of Chemical Regulation In The Guinea-Pig

It is evident from the table that there was a constant decrease in the metabolism as the air was warmed from o° to 350 C. The metabolism at o° was two and a half times that at 300, an increase as pronounced as is incurred as the result of severe muscular work. The animal at o° was not observed to move around any more than he did at 300. These results have been confirmed by Murschhauser.1 Other experiments confirmed Rubner in the view that the critical temperature, or the temperature of the minimum metabolism, lay at 330. At this point temperature had the least influence on total metabolism. When the temperature is raised from 300 there is at first no increase in the metabolism. This is due to the action of the apparatus for the physical regulation of body temperature. As the temperature rises the blood-vessels of the skin become dilated and the evaporation of water from the body is promoted. These factors tend to maintain the normal temperature of the organism by physical means. If the temperature of the air be high, so that the physical regulation be not sufficient to cool the body, then a supernormal temperature ensues. Such a febrile temperature raises the metabolism by warming the cells, as is seen in the table of the experiment in which the guinea-pig was exposed to a temperature of 400. The range of the physical regulation - that is, the period during which external temperature change does not alter metabolism - depends, according to Rubner, on the natural protections which an animal possesses which insure him against heat loss. These are two in number - the hairy covering and the thickness of the layer of subcutaneous fat.

1 Murschhauser: "Zeitschrift fur physiologische Chemie," 1912, Ixxix, 301.

Rubner has shown that the hair of the black cat, black lamb, rabbit, skunk, raccoon, mink, musk-deer, and sheep is of itself relatively light in weight, but that the fur contains a very large quantity of air. The whole of the fur covering of these animals consists of between 97.3 and 98.8 per cent, of air. The fur, therefore, really consists of air with between 1.2 and 2.7 per cent, of hair. The slight conductivity of the fur is principally dependent on this layer of stationary air. If an animal be covered with a fur containing this stagnant air, he will be better protected from loss of heat than if he had none, and also less susceptible to the influence of cold upon the surface of his skin. This protective covering therefore extends the range of the physical regulation.

Rubner1 gives the following experiment showing the influence of temperature on a small fasting dog with long hair:

Action Of Chemical Regulation In The Dog

One observation was made in this experiment on the dog which was not possible in the case of the guinea-pig, and that concerned the nitrogen excretion. The nitrogen excretion for twenty-four hours is not increased by exposing the dog to a temperature of 7.6°. The increased metabolism is entirely at the expense of fat. We have seen that this may also be true of work which may be accomplished at the expense of fat without raising the protein metabolism.

1 Rubner: "Die Gesetze des Energieverbrauchs," 1902, p. 105.

Reduced to terms of calories produced per kilogram of dog, the following results are obtained: