THE SECRETORY TISSUES AND ORGANS. 283 



some free surface of the Body, either that of the general 

 exterior or of some internal cavity, or into recesses commu- 

 nicating with such a surface. The 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 digestion. of food. The former group are 

 sometimes distinguished as excretions and the latter as secre- 

 tion* proper, but there is no real difference between them, the 

 organs and processes concerned being fundamentally alike in 

 each case. A better division is into transudata and secretions, 

 a transudation being a product which contains nothing which 

 did not previously exist in the blood, and only in such quan- 

 tity as might be derivable from it by merely physical processes; 

 while a secretion in addition to transudation elements contains 

 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 secretion in such a high 

 proportion that it must have been actively picked up and 

 conveyed there by the secretory tissues concerned. For in- 

 stance, 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 the kidney-cells to have a peculiar 

 power of removing that body from the liquids flowing near 

 them. This subdivision is also justifiable oh histological 

 grounds; wherever there is a secreting surface or recess it is 

 lined by cells, but these cells where transudata are formed (as 

 on the serous membranes) are mere flat scales, with little or 

 no protoplasm remaining in them (Fig. HB), while the cells 

 which line a true secreting organ are cuboidal, spherical, or 

 columnar, and still retain, with their high physiological activ- 

 ity, a good deal of their primitive protoplasm. 



Organs of Secretion. The simplest form in which a 

 secreting organ occurs (A, Fig. 104) 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 capillary 

 blood-vessels, c, on the other. The dividing membrane, b, 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 



