CHAPTER XVIII. 



THE SECRETORY TISSUES AND ORGANS. 



Definition. In a strict sense of the terms every pro- 

 cess in which substances are separated from the blood, 

 whether they be altered or unaltered, is "secretory" and 

 every product of such a separation is a " secretion;" in this 

 sense secretions would be separable into three classes. (1) 

 Liquids or gases transuding on free surfaces of the Body, 

 whether external or internal; (2) the liquids (lymph) 

 moistening the various tissues of the Body directly, filling 

 the interstices between them and not contained in definitely 

 limited cavities ; (3) all the solid tissues of the Body since, 

 after an early period of embryonic life, they are built up 

 from materials derived from the blood. Secretions would 

 thus come to include all constituents of the Body except 

 the blood itself but, while it is well to bear in mind that 

 the whole Body is in such a way derived from the blood, in 

 practice the term secretion is given a narrower connota- 

 tion, the solid tissues and the lymph being excluded; so that 

 a secretion is a material (liquid or gaseous) derived from the 

 blood and poured out on a free surface, whether that of the 

 general exterior or that of an internal cavity. Such true 

 secretions fall into two classes; one in which the product 

 is of no further use in the Body and is merely separated 

 for removal, as the urine; and one in which the product is 

 intended to be used, for instance as a solvent in the diges- 

 tion of food. The former group are sometimes distin- 

 guished as excretions and the latter as secretions proper, but 

 there is no real difference between them, the organs and pro- 

 cesses concerned being fundamentally alike in each case. A 

 better division is into transudata and secretions, a transuda- 



