3?8 DIGESTION 



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 dis- 

 covered 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 fer- 

 ments 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. 



Formation of Bile. 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 recognized 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 

 ascertain 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 the whole 

 intestinal tract out of gear. But after an artificial communication 

 has been made between the portal and the left renal vein or the 

 inferior cava, the portal may be tied and the animal live for months 



