November 24, 1911] 



SCIENCE 



701 



brane will ultimately become uniform. 

 Physics has demonstrated this. Is there 

 any difference in the results if the animal 

 membrane be living or dead? In some in- 

 stances it has been shown that it does. 

 Waymouth Reid introduced into the intes- 

 tine of a living animal a certain amount of 

 its own blood serum. The epithelial cells 

 of the alimentary tract were therefore in 

 contact on the one side with this blood 

 serum and on the other with capillary ves- 

 sels containing blood, the fluid of which 

 had the same composition as the serum in 

 the intestine. If the intestinal wall acted 

 like an ordinary dead membrane there 

 could be no passage of the serum from the 

 intestine to the blood by diffusion or osmo- 

 sis. It was found that the serum was 

 rapidly absorbed. This could not be due 

 to ordinary filtration because the pressure 

 in the intestine was less than in the capil- 

 laries. The conclusion was reached that 

 while known physical forces play a certain 

 part in absorption, there remains an un- 

 explained factor. Some, however, regard 

 this unexplained factor as the living cell 

 and that because of its living condition two 

 separated fluids of uniform composition 

 were made to unite against pressure. 



A somewhat similar example occurring 

 in the kidney may be referred to. Urine 

 contains a much higher percentage of urea 

 than does the blood, but in spite of that 

 the extremely weak solution of the urea in 

 the blood gives up its urea to the more 

 concentrated solution in the urine. Phys- 

 ical law will permit the passage of a sub- 

 stance from the stronger to the weaker so- 

 lution, but not the reverse. 



Cooperation is the key-note of physiol- 

 ogy. In no other science, perhaps, do we 

 have such striking examples. Since the 

 time of Sir Charles Bell, physiologists have 

 recognized the importance of the nervous 

 system in coordinating and regulating the 



various bodily functions. In compara- 

 tively recent years the realization has 

 grown that the harmonious adjustment of 

 the various tissues is not confined entirely 

 to the numerous reflexes through the nerv- 

 ous system, but that there is in addition 

 a regulation by chemical means through 

 the blood and other fluids of the body. 

 The development of our knowledge of the 

 internal secretions led Brown-Sequard to 

 the generalization that every tissue in the 

 body in the course of its normal function 

 gave material to the blood which was of 

 use in regulating the activity of other 

 tissues. This idea has been supported by 

 facts brought out in connection with the 

 study of the ductless glands of the body. 

 The generalization of Brown-Sequard has 

 been confirmed in a definite way by the in- 

 vestigations of Bayliss and Starling upon 

 secretin. They have demonstrated that 

 when hydrochloric acid is brought in con- 

 tact with the epithelial cells of the duode- 

 num a substance (secretin) is produced 

 which passes into the blood and is carried 

 to the pancreas and stimulates it to secrete 

 the pancreatic juice. This is a definite ex- 

 ample of a substance which, originating in 

 one tissue, is of direct aid in the function 

 of another tissue in a chemical way. Such 

 a substance has been designated by Star- 

 ling as a "hormone." 



The epithelial cells of the duodenum co- 

 operate in still another very striking man- 

 ner. "When the pancreatic juice is secreted 

 its proteolytic enzyme is in the form of 

 trypsinogen — an inert substance. As soon 

 as it comes in contact with the duodenum, 

 the trypsinogen is activated or converted 

 into trypsin, which has the power of acting 

 vigorously upon proteid material. The 

 substance which causes this activation is 

 known as enterokinase. It would thus ap- 

 pear that the duodenal cells are doubly co- 

 operative in assisting other tissues in a 



