REGULATION OF THE ANIMAL TEMPERATURE. 311 



passing through these organs becomes warmer instead of cooler, and 

 receives heat from the changes taking place in the glandular tissue. 



Equalization of Temperature by the Circulation. As the production 

 of heat is a local process in each separate organ or tissue, varying in 

 intensity with the nature of the nutritive changes in different parts, the 

 blood, as we have seen, acquires a higher temperature in some organs 

 than in others ; and in the lungs and skin its heat actually diminishes 

 instead of increasing. If it remained at rest, these differences of tem- 

 perature would no doubt be more marked than they are at present. But 

 as the blood is in constant motion, passing from the circumference to 

 the centre, and being again distributed from the centre to the circum- 

 ference, the effect of this movement of circulation is to equalize to a 

 considerable degree the temperature of different parts of the body. The 

 venous blood coming from the general integument with a diminished 

 temperature is mingled with that of the muscular system, which has 

 become warmed during its capillary circulation. The blood of the 

 hepatic veins, which is the warmest of all, joins the current of the inferior 

 vena cava, returning from the pelvic organs and the inferior extremities. 

 This is again mingled, at its entrance into the right cavities of the heart, 

 with the slightly cooler column of blood descending from the head and 

 upper extremities by the superior vena cava. The whole volume of the 

 blood then passes through the lungs, with the effect of still further 

 moderating its temperature ; and the arterial blood is then distributed 

 to the various parts of the body, to gain warmth in some of them and to 

 lose it in others, and again mingled after a few seconds at the centre 

 of the circulation. Thus the superabundant heat of certain organs, 

 where its production is most active, is constantly transferred to others 

 by the moving column of the blood ; and a certain equilibrium or standard 

 of temperature is thus established for the body as a whole. It is found, 

 by the observations of Jurgensen, that this standard temperature for 

 the human body, as measured in the rectum, varies within very narrow 

 limits, from day to night, and even at successive periods of each division 

 of the twenty-four hours. These normal fluctuations are no doubt owing 

 to the greater or less activity, at different times, of different internal 

 organs ; the total amount of heat produced being increased or dimin- 

 ished with the preponderating influence of organs in which it is more or 

 less rapidly generated. 



Regulation of the Animal Temperature. 



A certain temperature is .not only the result of the vital actions; it is 

 also necessary to their accomplishment. Even in the vegetable king- 

 dom this temperature, which varies within moderate limits in different 

 kinds of plants, is requisite for all the phenomena of growth and vitality. 

 A seed sown in the most productive soil does not germinate until it 

 feels the influence of the necessary warmth ; and its germination is also 

 impossible if it be exposed to a heat which is too intense. The degrees 

 both of heat and cold which favor or arrest the functions of vegetation 



