260 THE HUMAN BODY. 



tion being a product which contains nothing which did 

 not previously exist in the blood, and then in such quantity 

 as might be derivable from it by merely physical processes; 

 while a secretion in addition to transudation elements con- 

 tains a specific element, due to the special physiological 

 activity of the secretory organ; being either something 

 which does not exist in the blood at all or something which, 

 existing in the blood in small quantity, exists in the secre- 

 tion in such a high proportion that it must have been 

 actively picked up and conveyed there by the secretory 

 tissues concerned. For instance, the gastric juice contains 

 free hydrochloric acid which does not exist in the blood; 

 and the urine contains so much urea that we must suppose 

 its cells to have a peculiar power of removing that body 

 from the liquids flowing near them. This subdivision is 

 also justifiable on histological grounds ; wherever there is 

 a secreting surface it is covered with cells, but these where 

 transudata are formed (as on the serous membranes) are 

 mere flat scales, with little or no protoplasm remaining in 

 them, while the cells which line a true secreting organ are 

 cuboidal, spherical, or columnar, and still retain, with 

 their high physiological activity, a good deal of their primi- 

 tive protoplasm in a but slightly modified state. 



Organs of Secretion. The simplest form in which a 

 secreting organ occurs (A, Fig. 88) is that of a flat membrane 

 provided with a layer of cells, a, on one side (that on which 

 the secretion is poured out) and with a network of capil- 

 lary blood-vessels, c, on the other. The dividing mem- 

 brane, 1), is known as the basement membrane and is usually 

 made up of flat, closely fitting connective- tissue corpuscles; 

 supporting it on its deep side is a layer of connective tissue, 

 d, in which the blood-vessels and lymphatics are supported. 

 Such simple forms of secreting surfaces are found on the 

 serous membranes but are not common; in most cases an 

 extended area is required to form the necessary amount of 

 secretion, and if this were attained simply by spreading out 

 plane surfaces, these from their number and extent would 

 be hard to pack conveniently in the Body. Accordingly in 

 most cases, the greater area is attained by folding the 



