420 MANUAL OF PHYSIOLOGY. 



ferent authors upon this point can be removed, and a general idea 

 of the manufacture of fats from various food-stuffs can be gained, 

 by bearing in mind the assimilative and secretive functions of the 

 protoplasm. There can be no doubt whatever that the active 

 protoplasm of many parts and organs if properly nourished can 

 manufacture fat. As examples, we may take the liver and mam- 

 mary cells, and those connective tissue cells which have no great 

 nutritive duty to attend to. This fat production by the proto- 

 plasm may be regarded as a secretion of fat, though only in one 

 of the examples given does it appear externally as a definite 

 secretion milk. We cannot scrutinize the chemical methods by 

 which this change is brought about in protoplasm, any more than 

 those which give rise to any other secretion. We know that pro- 

 toplasm uses as pabulum, albumin, fat, and carbohydrate, and 

 we have no reason to doubt that the proportion of these materials 

 found to form the most nutritious diet for the body generally, is 

 also the proportion in which protoplasm can best make use of 

 them. Probably such cells as part with a material containing 

 nitrogen such as mucin-yielding gland cells require a greater 

 proportion of pabulum containing nitrogen (albumin) for their 

 perfect function. Probably those cells which produce a large 

 quantity of non-nitrogenous material do not require more nitro- 

 gen than is necessary for their perfect reintegration as nitrogenous 

 bodies. But for their active function, i.e., the manufacture of 

 their secretion, they only require a pabulum which contains tin- 

 same chemical elements as are to be found in the output. In the 

 case of fat-formation, then, a supply of fat or carbohydrate ought 

 to suffice if accompanied by a small amount of albuminous sub- 

 stance. If these non-nitrogenous substances be withheld, the 

 protoplasm can no doubt obtain the quantity of carbon, hydro- 

 gen, and oxygen requisite to manufacture fat from albumin, and 

 thus a large amount of nitrogen will be wasted. 



There is nothing in the foregoing statement that is not in ac- 

 cord with the results of practice and experiment. 



Fat cannot be produced without nitrogen in the diet, because 

 the fat-manufacturing protoplasm cannot live without nitrogen, 

 which is absolutely necessary for its own assimilative reintegra- 



