GENERAL METABOLISM. 643 



nitrogen equilibrium before the period of gestation. From this fact it 

 follows that the developing fetus does not live at the expense of the organ- 

 ism of the mother, but to a certain extent is included in the general nourish- 

 ment. The organism of the pregnant mother utilizes to better advantage 

 than usual the substances contained in the food and especially the protein. 



In discussing the replacement of one foodstuff by another and the posi- 

 tion of each in animal economy, we have considered the suitability of each 

 kind of food for definite functions. We have seen that the carbohydrate 

 decomposition is proportional to the amount of muscular work, and that 

 fats and also albumins can replace carbohydrates as sources of energy. 1 



It remains for us to mention briefly the influence of certain external 

 conditions upon metabolism. Above all, the influence of the surround- 

 ings is of interest. Up to the present time the effect of temperature has 

 alone been studied to any extent. In this respect the poikilothermous 

 animals are very different from the homothermous ones. In the former, 

 metabolism runs parallel to the variations in temperature, i.e., it decreases 

 with a fall in the external temperature and increases with a rise in tem- 

 perature. This can be demonstrated very clearly by studying the respi- 

 ratory exchange. The warm-blooded animals, on the other hand, 

 behave quite differently. With them metabolism increases with falling 

 temperature and decreases with rising temperature. This is due to the 

 fact that the warm-blooded animals seek to keep their body-temperature 

 the same, irrespective of the external conditions. The loss of heat caused 

 by a fall in the external temperature is compensated by an increased 

 metabolism. The muscles are the principal seat of this change in the extent 

 of the metabolism. If the muscular activity which may be expressed 

 by movements, by shivering, or also by muscular tensions, is prevented 

 by curare poisoning or by severing the spinal cord high up, the heat 

 regulation ceases. The principle that the metabolism of homothermous 

 animals increases with falling temperature and conversely diminishes with 

 rising temperature holds only in part. It has been shown that a consid- 

 erable rise of temperature also has a similar effect upon the warm-blooded 

 animals as upon the cold-blooded ones; the metabolism increases so that 

 there is an increased production of heat against which the organism seeks 

 to protect itself by giving off more heat. The chief difference, then, 

 between the homothermous animals and the poikilothermous ones is their 

 opposite behavior with regard to a fall in temperature. 



1 There are a great many experiments concerning metabolism under varying condi- 

 tions. We cannot consider them here, because in most cases it is difficult to establish 

 the exact effect of the different factors. We would refer to the influence of high altitude 

 upon metabolism, and of a sea-shore climate. Cf. N. Zuntz, A. Loewy, F. Miiller, and 

 W. Caspari: Hohenklima und Bergwanderungen in ihrer Wirkung auf den Menschen. 

 Bong & Cie, 1906. A. Jaquet and R. Stahelin: Arch, exper. Path. Pharm. 46, 274 

 (1901). Loewy and Muller: Pfliiger's Arch. 103, 1 (1904). 



