THE WORK OF MUSCLES AND GLANDS 45 



of the action of all the tissues upon it. But we shall find 

 that internal secretion is a function much more clearly 

 attributable to certain organs than toothers and most 

 evident in connection with a few small structures like the 

 thyroid and the adrenal, to which later reference must 

 be made. To have an internal secretion an organ need 

 not be a typical gland. No duct will be required to carry 

 such materials as its cells turn over to the blood-stream. 

 In some cases organs believed to work along these lines are 

 spoken of as ductless glands. A word recently introduced 

 as an equivalent of the term " internal secretion" should 

 be given. It is the word " hormone," meaning a chemical 

 messenger, a very convenient and suggestive expression. 



Absorption and Secretion. Gland-cells have been said 

 to draw upon the blood or the lymph for their raw mate- 

 rial and to manufacture their secretions therefrom. In 

 this process something enters the deeper boundary of the 

 cell layer and in a more or less transformed state it is 

 later discharged from the exposed surface. It is helpful 

 to compare this operation with what takes place in the 

 intestine when the products of the digestive cleavage are 

 being removed to the circulation. When absorption is 

 going on it is the exposed ends of the cells which receive 

 dissolved substances and their deeper borders which are 

 discharging to the fluids that underlie them. Such a 

 process has been well called " reversed secretion," and 

 there is the same possibility of an extensive making over 

 of the transferred material in this case as in the other. 

 In other words, the digestive products which are last 

 detected in the intestine are not necessarily those which 

 will be dealt out to the blood by the cells of the absorbing 

 membrane. Both secretion and absorption are phenom- 

 ena which can be completely carried out only by living cells. 

 Each is probably promoted by a definite application of 

 energy on the part of the cells concerned. In either case 

 it is possible that there may be some transfer of material 

 through the clefts between the cells as well as through 

 the cell bodies. 



