84 PRINCIPLES OF PHYSIOLOGY 



which special substances are formed. The 

 cell gradually is filled with these substances and 

 then it bursts, liberating its contents, or in 

 some cases there seems to be a gradual removal 

 from the cell of the products of its secretion. 

 The matters thus collected from innumerable 

 cells become the secretion of the gland. We 

 may take as an example the secretion of 

 saliva. The cells of the salivary gland elabor- 

 ate the materials that form the secretion. In 

 particular, they form a peculiar body, known 

 as ptyalin, belonging to the class of ferments. 

 There is no ptyalin in the blood. It is formed 

 in the cells of the gland. Nor is it at once 

 formed from materials supplied by the blood. 

 There is at least one antecedent substance, 

 probably more than one, marking stages in the 

 gradual formation of ptyalin. In like man- 

 ner, the cells of the mammary gland elaborate 

 the complex fluid, milk, and those of the 

 pancreatic gland the remarkable bodies found 

 in the secretion of that gland, all of which are 

 ferments. Secretion, however, is essentially 

 a form of growth depending on the activities 

 of the secreting celL 



