268 FUNCTIONS OF NUTRITION. 



in others ; while in the lungs and skin its heat diminishes instead of 

 increasing. If the blood remained at rest, these differences would be 

 still more marked. But as it is in constant motion, from the circum- 

 ference to the centre, and again from the centre to the circumference, 

 the effect of the circulation is to equalize, in great measure, the temper- 

 ature of different parts. The blood coming from the general integu- 

 ment with a diminished temperature is mingled with that of the mus- 

 cular 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, with a somewhat lower temperature, 

 returning from the pelvic organs and the inferior extremities. It is 

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

 the blood descending from the head and upper extremities by the supe- 

 rior 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 distributed to the various parts of the body, to gain 

 warmth in some and to lose it in others, and to be again mingled after 

 a few seconds at the centre of the circulation. The superabundant 

 heat of certain organs, where its production is most active, is con- 

 stantly transferred to others by the moving column of the blood ; and 

 a certain equilibrium of temperature is thus established for the body as 

 a whole. In the observations of Jurgensen, this standard temperature, 

 as measured in the rectum, was found to vary, within narrow limits, 

 from day to night, and even at successive periods in the twenty-four 

 hours. These fluctuations are no doubt due to the varying functional 

 activity of different parts ; the total amount of heat produced being 

 increased or diminished 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 vegetables this 

 temperature, which varies within moderate limits in different plants, is 

 requisite for all the phenomena of growth and vitality. A seed sown 

 in the most productive soil does not germinate except under the influ- 

 ence 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 have 

 been in many instances accurately determined. According to Sachs, 

 the limits of germination for wheat and barley are between 5 and 38 

 C., and for Indian corn between 9 and 42. The irritability and 

 periodic movements of the sensitive-plant do not show themselves 

 unless the temperature of the surrounding air be above 15. In air 

 at 48 to 50, on the other hand, the leaflets become rigid in a few 

 moments, though they may afterward recover if the temperature be 

 moderated ; while a heat of 52 permanently destroys their vitality. 

 Thus no vegetative function can come into activity, unless the temper- 



