592 GENERAL METABOLISM [CH. XXXIX. 



The most striking examples of the formation of fat by intracellular metabolic 

 processes is seen in fatty degeneration, and in that special form of this degeneration 

 that occurs in the formation of milk. The blood contains a mere trace of fat, so 

 milk formation is no mere filtration process. The food may, as in the case of cows, 

 contain little or no fat. 



Feeding with gelatin. A diet containing gelatin alone will not support life. 

 This fact is somewhat remarkable when one considers the closely allied chemical 

 nature of gelatin and proteids. When gelatin alone is given, the body wastes, and 

 the urea excreted is diminished as in inanition. If an enormous amount of gelatin 

 is given the urea increases. Gelatin, however, like carbohydrates and fats, appears 

 to be a " proteid-sparing " food, and if given mixed with proteids seems to protect 

 the proteids from oxidation. Gelatin can thus be substituted for a part of the pro- 

 teid in the food. 



Feeding with "peptones" In the present day, when artificially digested foods 

 are so much employed, it is of great importance that their nutritive value should be 

 known. Here experimental and clinical evidence coincide in a most favourable way 

 in relation to their nutritive value. Albumoses and the preparations called peptone 

 in commerce, which are in reality mainly albumoses, have the same nutritive value 

 as meat 



Concentrated or artificially digested foods of ,_this kind (Witte's peptone, 

 plasmon, somatose, etc.) must naturally be distinguished from beef-tea and 

 extracts of meat of which there are so many in the market, but which are mere 

 stimulants and are valueless for nutrition (see p. 469). 



Effect of Varying External Conditions on Exchange of Material. 



Effect of atmospheric temperature. In warm-blooded animals the effect of a 

 low surrounding temperature is to increase katabolism, or combustion in the body ; 

 the body loses more heat, and therefore more must be produced to keep the animal's 

 temperature within normal limits. The effect of a rise of atmospheric temperature 

 is the reverse. In cold-blooded animals, i.e. , animals whose temperature varies with 

 that of the surrounding atmosphere, a rise or fall of the latter is accompanied respec- 

 tively with a rise or fall of combustion in the body. Pembrey has shown that warm- 

 blooded animals in an embryonic condition are practically cold-blooded ; that is, 

 their metabolism, body temperature, and the external temperature vary directly 

 the one with the others. 



Alterations of body temperature. If the changes of the external temperature 

 are so great as to cause a rise (as in steam-baths) or a fall (as in hibernation) of body 

 temperature, the metabolic changes are increased and decreased respectively, as in 

 cold-blooded animals. 



Effect of removal of blood from the body. The chief effect of a removal of blood 

 from the body is the speedy formation of new blood-corpuscles. The intake of 

 oxygen and discharge of carbonic acid are lessened, and the output of urea is 

 increased. The menstrual flow and epistaxis in strong, healthy people cause no 

 alteration in exchange of material. 



Exchange of Material in Diseases. 



Fever. Fever is a condition in which the temperature of the 

 body is raised above the normal, and the degree to which it is raised 

 is a measure of the intensity of the febrile condition. A rise of 

 temperature may be produced either by increased production of heat, 

 due to the increase of katabolic processes in the body, or to a 

 diminished loss of heat from the body. A mere increase in the 

 production of heat does not necessarily produce fever. By administer- 

 ing an excess of food, combustion is increased in the body ; but in 

 the healthy individual this does not produce a rise of temperature, 

 because pari passu with the increased production there is increased 

 loss of heat. Similarly, diminution in the loss of heat, such as 



