36 



A PRIMER OF BIOLOGY 



glands which line or open into the mouth cavity, 

 and. after being swallowed, is mixed with other 

 secretions derived from glands which line or open by 

 ducts into the alimentary canal. By these secretions 

 the food is altered in character, one secretion acting 

 on one constituent, another secretion acting on 

 another. In their progress through the canal the 

 altered food-stuffs, now made assimilable, are slowly 

 absorbed by vessels which permeate the walls of 

 the canal, and are by them transferred, directly or 



indirectly, to the 

 B tissues. 



In order that 

 we may obtain 

 some conception 

 of the nature and 

 mode of action 

 of a digestive 

 secretion, we may 

 select that formed 

 by the glands 

 which open into 



mouth-cavity the salivary glands. The essen- 

 constituent of such a digestive secretion is an 

 organic body know r n as an enzyme or ferment. 

 On examining a portion of one of the salivary 

 glands under the microscope, we find (Fig. 13) 

 that it is composed of an immense number of 

 delicate protoplasmic cells, which, before the gland 

 begins to secrete, are very granular. These cells are 

 in communication, by means of minute intercellular 

 channels, with one or more ducts which open into 

 the mouth-cavity. As secretion proceeds, the 

 granulation in the cells gradually disappears, and 

 from the mouths of the ducts there exudes a colourless 



FIG. 13. Cells from a salivary gland 

 A, before ; B, after secretion. 



the 

 tial 



