CHAPTER XXII 

 METABOLISM 



The word metabolism has been used once or twice 

 before this time to signify the sum of the chemical 

 changes taking place in the body. The German equiva- 

 lent is very significant: it is the word Stoffwechsel, which 

 means " transformation of matter." We exclude from 

 the subject the digestive reactions which go on in the 

 alimentary tract; strictly speaking these do not occur in 

 the body but near its surface where it is infolded. Our 

 starting point must be the absorption of the digestive 

 products, the final topic of Chapter XV. Most of what 

 we have now to discuss might be entitled The History 

 of the Food after Absorption. 



Metabolism of Fat. The story is most simple in the 

 case of the fats. It will be recollected that these com- 

 pounds are decomposed in the intestine but reconstructed, 

 it appears, in the very act of passing through the epi- 

 thelial wall. Drops of fat occur in the lymph of the 

 mesentery and give it the milky character which long 

 ago fixed the name of lacteals upon these vessels. It is 

 known that most of the fat entering the circulation after 

 a meal traverses the thoracic duct. This was ascertained 

 by observing cases in which the duct had been severed so 

 that the lymph stream escaped through a wound. After 

 a meal containing a known amount of fat about two- 

 thirds of it could be recovered in the collected lymph. 



The fat which is generated in the intestinal wall from 

 the cleavage-products of fat previously fed and digested 

 is not a mere reproduction of the original fat. It has a 

 constant composition proper to the species and inde- 

 pendent of variations in the nature of the fat furnished 

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