DIGESTION 327 



We have spoken more than once of the gland-cells as 

 manufacturing their secretions. It is an idea that rises 

 naturally in the mind as we follow with the microscope the 

 traces of their functional activity. And when we compare 

 the composition of the digestive juices with that of the 

 blood-plasma and lymph, the suggestion that the glands 

 which produce them are not merely passive filters, but 

 living laboratories, acquires additional strength. It is 

 evident that everything in the secretion must, in some form 

 or other, exist in the blood which comes to the gland, and 

 in the lymph which bathes its cells. No glandular cell, if 

 we except the leucocytes, which in some respects are to be 

 considered as unicellular glands, dips directly into the blood ; 

 everything a gland-cell receives must pass through the walls 

 of the bloodvessels. So that anything which we find in the 

 secretion and do not find in the blood must have been 

 elaborated by the gland from raw material brought to it by 

 the latter. 



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 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. 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. 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 FIG. 106. H^EMATOIDIN. 

 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 recognised 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 absorp- 

 tion 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, as was shown 

 by Ludwig and Fleischl in the dog. If the thoracic duct and the 

 bile-duct are both ligatured, no bile-acids or pigments appear in the 



