218 TEXT-BOOK OF PHYSIOLOGY. 



vessels and carried direct to the liver. Even though there is no appreciable 

 increase in the amount of protein in the portal blood during digestion, there 

 is every reason to think that this is the route by which it reaches the general 

 circulation. Ligation of the thoracic duct does not interfere to any appre- 

 ciable extent with protein absorption nor with the normal elimination of urea 

 nor with the weight of the animal. 



The foregoing statements are based on the view that the final stage in 

 the digestion of proteins is the formation of peptones. There are reasons 

 however for believing that the change is more far-reaching and complete, and 

 that the peptones in turn are disintegrated and reduced to still less complex 

 bodies represented by polypeptids, peptids, and even amino-acids. (See 

 page 189.) The extent to which this disintegration proceeds will doubtless 

 depend on the quantity and variety of proteins consumed. 



If the reduction of the protein molecule to this fragmentary condition is 

 the outcome of protein digestion, as recent investigations indicate, then the 

 problem of absorption is transferred to these fragmentary bodies rather than 

 to the peptone molecule. Inasmuch as the presence of the peptids and the 

 amino-acids in the blood of the portal vein has not been demonstrated 

 beyond question, the supposition is that after their absorption by the intes- 

 tinal epithelium, they are synthesized and a protein molecule constructed, 

 similar to, if not identical with, the plasma-albumin. This view renders 

 it much easier to understand how out of the different proteins, vary- 

 ing widely in their composition, the specific proteins of the blood are con- 

 structed. It is only necessary to assume that the epithelial cell selects from 

 the variety of fragments presented to it, only those which are necessary to 

 the formation of the plasma-albumin and the plasma-globulin, and to syn- 

 thesize them to these characteristic compounds. 



The plasma-albumin thus becomes the common protein out of which 

 each tissue constructs the particular kind of protein characteristic of it, 

 thus bringing about repair and growth. Whether this is accomplished by 

 the simple incorporation of the protein molecule directly or whether it must 

 be first reduced to amino-acids before the tissues can construct their own 

 protein is unknown. That the plasma-albumin bears an intimate relation 

 to the nutritive activities of the tissues is apparent from the decline in the 

 general nutrition and a marked loss of body weight, when in consequence of 

 diseases of the kidney it escapes in the urine. An alternate assumption 

 however is conceivable, viz., that the amino-acids, though not readily demon- 

 strable in the blood of the portal vein, are nevertheless absorbed as such, pass 

 through' the liver, enter the blood of the general circulation, and are carried 

 direct to the tissue-cells in which they are directly synthesized into the form 

 of protein characteristic of them. The plasma-albumin might then be 

 regarded as a protein surplus to be called upon if the protein ingested 

 should be insufficient. 



Many facts in the physiologic chemistry of the body raise the question 

 as to what percentage of the amino-acids is utilized for tissue repair and 

 growth and what percentage for heat production. If the protein require- 

 ments of Chittenden, viz., 58 to 60 grams only, are necessary for repair and 

 growth, then approximately one-half the amino-acids ordinarily produced 

 are used for heat production. The manner of disposal of these unused 



