59O GENERAL METABOLISM. [CH. xxxix. 



metabolism is raised. Carbohydrate food is thus when given with other 

 foods both fat-sparing and proteid-sparing. The formation of fat from 

 carbohydrates was first observed in pigs by Lawes and (Gilbert, and has since 

 been confirmed by numerous investigators. 



One of the most important instances of the carbohydrate origin of fat is 

 the formation of bees' -wax. 



Instances of the formation of fat from proteids are (i) the laying on of 

 fat in carnivorous animals ; (2) the formation of adipocere, a wax-like 

 material which forms in the muscles of corpses buried in damp soil, or 

 allowed to remain in water ; (3) the gradually increasing quantity of fat in 

 old cheeses. 



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 con- 

 tains 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 proteid 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. 



Artificially digested foods of this kind (Witte's peptone, plasmon, 

 somatose, &e.) 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. 466). 



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 respectively 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 tempera- 

 ture 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. 



