v o A MANUAL OF PHYSIOLOGY 



any organ with an active metabolism, the venous blood leaving it is 

 warmer than the arterial blood coming to it. The temperature of 

 the blood passing from the levator labii superioris muscle of the 

 horse during mastication may be sensibly higher than that of the 

 blood which feeds it ; the blood in the vena profunda femoris, and 

 in the crural vein of a dog with the leg wrapped in cotton-wool, is 

 warmer by *i to '3 than the blood of the crural artery. This 

 difference of temperature is due to the heat produced in the muscles, 

 and it is not difficult to show that the difference ought to be of this 

 order of magnitude. The quantity of blood in a 7 kilo dog is about 

 J kilo ; \ of this, or kilo, is in the skeletal muscles, and the 

 average circulation-time through them may be taken as ten seconds. 

 Six times in the minute, or 360 times in the hour, -J kilo of blood 

 passes through the muscles, and is heated on the average by '2. If 

 we take the specific heat of blood as about equal to that of water, 



this represents a heat-production of r- x -- x 1,000, or 9,000 small 



o IO 



calories per hour. Now, the total heat-production of a 7 kilo dog 

 is about 19,000 small calories per hour, of which somewhat less than 

 one-half is probably formed in the skeletal muscles. 



The blood of the inferior vena cava at the level of the kidneys 

 may be 'i colder than that of the abdominal aorta, but is 

 warmer than the blood of the superior cava. The right heart, 

 therefore, receives two streams of blood at different temperatures, 

 which mingle in its cavities. A controversy was long carried on as 

 to the relative temperature of the blood of the two sides of the 

 heart; but the researches of Heidenhain and Korner have shown 

 that a thermometer passed into the right ventricle through the jugular 

 vein stands, as a rule, slightly higher than a thermometer introduced 

 through the carotid into the left ventricle. They consider that the 

 method gives not so much the temperature of the blood in the two 

 cavities as that of their walls. The thin-walled right ventricle, 

 according to them, is heated by conduction from the warm liver, 

 from which it is only separated by the diaphragm, while the left 

 ventricle loses heat to the cooler lungs. They deny that the differ- 

 ence of temperature is caused by cooling of the blood in its passage 

 through the pulmonary capillaries, for even when respiration is sus- 

 pended, they find a difference of temperature between the two sides 

 of the heart. Under ordinary circumstances, they say, the inspired 

 air is already heated almost to body temperature before it reaches the 

 alveoli. But, while this is the case, it is possible that much of the 

 water- vapour required to saturate it is evaporated from the alveolar 

 walls, and a fall of less than y 1 ^ in the temperature of the blood 

 passing through the lungs would account for all the heat lost by the 

 expired air. Jf half of the loss took place in the upper air-passages, 

 less than ^ would be sufficient. A slight difference of temperature 

 in the blood of the two ventricles might be caused, even in the 

 absence of respiration, by the heat developed in the cardiac muscle 

 itself during contraction, a large proportion of which must be con- 

 veyed by the blood of the coronary veins into the right side of the heart. 



