DIGESTION 355 



blood ; everything a gland-cell receives must pass through the 

 walls of the bloodvessels. (But see footnote on p. 14.) So that 

 anything which we find in the secretion and do not find in the 

 blood must have been elaborated by the gland epithelium (or 

 by the capillary endothelium) from raw material brought to it 

 by the blood. 



Take, for example, the saliva or gastric juice. These liquids 

 both contain certain things that also exist in the blood, but in 

 addition they contain certain things specific to themselves : 

 mucin in saliva, hydrochloric acid in gastric juice, ferments in 

 both. It is true that a trace of pepsin and a trace of a diastatic 

 ferment may be discovered in blood ; but there is no reason 

 whatever to believe that this is the source of the pepsin of the 

 gastric juice, or the ptyalin of the salivary glands, except, 

 perhaps, in animals like the cat, whose saliva contains a diastase 

 in still smaller concentration than the serum (Carlson). On the 

 contrary, it is possible that the ferments of the blood may be in 

 part absorbed from the digestive glands, the rest being formed 

 by the leucocytes and liberated when they break down. The 

 liver affords an even better example of this ' manufacturing ' 

 activity of gland-cells, and many facts may be brought forward 

 to prove that the characteristic constituents of the bile, the bile- 

 pigments and bile-acids, are formed in the liver, and not merely 

 separated from the blood. Bile-pigment has indeed been recog- 

 nized in the normal serum of the horse, and bile-acids in the chyle 

 of the dog, but only in such minute traces as are easily accounted 

 for by absorption from the intestine. Frogs live for some time 

 after excision of the liver, but no bile-acids are found in the 

 blood or tissues. But if the bile-duct be ligatured, bile-acids and 

 pigments accumulate in the body, being absorbed by the 

 lymphatics of the liver (Ludwig and Fleischl). If the thoracic 

 duct and the bile-duct are both ligatured in the dog, no bile- 

 acids or pigments appear in the blood or tissues. Wertheimer 

 and Lepage state that bile or bilirubin injected into a bile-duct 

 appears sooner in the urine than in the lymph of the thoracic 

 duct, and therefore conclude that the bloodvessels are the most 

 important channel of absorption. This conclusion, however, 

 cannot be accepted until it is shown that in these experiments 

 the injection did not cause rupture of some of the hepatic 

 capillaries and direct entrance of the bile-pigment into the blood. 

 It is not improbable that the pressure attained by the bile in 

 the bile-capillaries is a factor in determining the path by which 

 it is absorbed, and that when the pressure rises beyond a certain 

 limit it may pass both into the bloodvessels and into the 

 lymphatics. In mammals life cannot be maintained for any 

 length of time after ligature of the portal vein, since this throws 



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