CHAPTER VII 

 SECRETION IN GENERAL 



ALL tissues of the body produce certain chemical changes as a result of 

 their protoplasmic activity. But in certain cells chemical elaborations have 

 come to be the chief function, the cells have been differentiated in that direc- 

 tion, and the name secreting tissue or gland tissue is applied. The end result 

 of metabolism in gland tissue is the extrusion on the free borders of the cells 

 of the products of their metabolism. The products are known as secretions 

 and the process itself is the act of secretion. Certain secretions which are 

 in the nature of waste products to the body as a whole, such as urine in the 

 kidney, are often called excretions, but the use of the term should not be allowed 

 to confuse the general similarity of this to other secretions as regards the 

 physiological changes involved in its production. 



Most secretions accomplish some definite office in the economy of the 

 body. Those that are discharged on some free mucous surface, as the saliva, 

 gastric juice, tears, etc., are called external, or true secretions, or merely secre- 

 tions. Substances that are discharged back into the blood stream later to 

 influence the metabolism of tissues other than the ones which produced them 

 are called internal secretions. 



Gland cells, like other tissues, draw their nourishment from the blood 

 and lymph. The product or secretion of gland cells may, in fact usually 

 does, contain some of the substances found in the blood, but there are also 

 present new materials elaborated by the cells, and even where the same sub- 

 stance exists both in the secretion and in the blood and lymph it can make 

 its appearance in the secretion only under the control of the protoplasm of 

 the gland cells. The saliva secreted by the salivary cells, for example, con- 

 sists of about 98 to 99 per cent, water containing in solution small quantities 

 of certain salts, also found in the lymph, and a small percentage of the en- 

 zyme, ptyalin. This enzyme is peculiar to the salivary secretion and is manu- 

 factured by the salivary-cell protoplasm. As is well known, it acts vigorously 

 in extreme dilution, hence the high per cent, of water in the secretion. The 

 passage of water from a solution as concentrated as blood plasma to a solu- 

 tion as dilute as saliva requires a high amount of osmotic energy, an amount 

 that can be supplied only from the chemical energy liberated by the cell in 

 its protoplasmic activity. After the removal of the special organ by which 

 each secretion is manufactured, the secretion is no longer formed. Cases 

 sometimes occur in which the secretion continues to be formed by the natural 



