MODE OF PRODUCTION OF ANIMAL HEAT. 431 



recently passed through a part of the body where it has had an 

 opportunity of losing or gaining heat, its temperature varies, but 

 only within narrow limits. The mean temperature of the blood is 

 higher than that of any other tissue. The blood in the hepatic 

 capillaries is the warmest in the body. This reaches 40.73 in the 

 dog, or nearly two degrees higher than that in the aorta of that 

 animal. The cool blood from the extremities and head mingling 

 in the right side of the heart with the unusually warm blood from 

 the liver keeps the blood going to the lungs at the standard tem- 

 perature. The blood in the left side of the heart is a little cooler 

 than the right, probably because the latter lies on the warm liver, 

 as is proved by the substitution of a cold object for this organ, 

 when the temperature is reversed, and the blood on the right side 

 becomes colder than the left. It is not because the blood is cooled 

 going through the lungs, for the heat used in warming the respired 

 air is given off by the nose and other air passages, and not by the 

 alveoli of the lungs. 



III. The temperature of an individual organ varies with the 

 state of its activity. During their activity the glands, muscles, 

 etc., receive more blood, undergo more active chemical changes 

 and become warmer. 



MODE OF PRODUCTION OF ANIMAL HEAT. 



It has already been indicated that the general effect of the tissue 

 change of the body is a kind of combustion in the tissues of certain 

 substances obtained from the vegetable kingdom, viz., proteid, fat, 

 carbohydrate, etc. The combustible substances are capable of 

 being burned in the open air, or made to unite with oxygen so as 

 to produce a certain amount of heat, being thus converted into 

 CO 2 and H 2 O. In the body the oxidation goes on in a gradual or 

 modified way, and the end products of the process can be recog- 

 nized as CO 2 eliminated from the lungs, and as water and urea 

 got rid of by the kidneys. The general tendency of the chemical 

 changes in the tissues is such as will set free energy in the form 

 of heat. 



The amount of heat that any substance is capable of giving off 

 corresponds to the amount of energy required for the formation 



