298 THE HUMAN BODY. 



indirectly controlled by the nervous system, through the in- 

 fluence of the latter upon the circulation of the blood; they 

 are however dependent on the structure of the cells concerned, 

 so that the characters of the transudata and their quantity are 

 altered when the cells are diseased. After death, too, the 

 process of dialysis through such cells is different from that 

 during life, for the living cell controls to a certain extent the 

 nature and amount of the substances which it will allow to 

 pass through it. The cells lining such surfaces are not, how- 

 ever, secretory tissues in any true sense of the word. In 

 other cases the lining cells are thicker, and more actively 

 concerned in the process; they are then usually spread over 

 the recesses of a much folded membrane, so that the whole 

 is rolled up into a compact organ called a gland, the secre- 

 tion of which may contain only transudation elements (as 

 for example that of the lachrymal glands which form the 

 tears) or may contain a specific element, formed in the 

 gland by its cells, in addition to transudation elements. 

 In both cases the activity of the organ may be influenced 

 by the nervous system, usually in a reflex manner (e.g. the 

 watering of the eyes when the eyeball is touched and the 

 saliva poured into the mouth when food is tasted), but may 

 also be otherwise excited, as for example the flow of tears 

 under the influence of those changes of the central nervous 

 system which are associated with sad emotions, or the water- 

 ing of the mouth at the thought of dainty food. The nerves 

 going to such glands, besides controlling their blood-vessels, 

 act upon the gland-cells; one set governing the amount of 

 transudation of water and salines which shall take place 

 through them, and another (in the case of glands producing 

 secretions with one or more specific elements) controlling the 

 production of these, by starting new chemical processes in 

 the cells by, which a substance built up in them during rest 

 is converted into the specific element, which is soluble in and 

 carried off by the transudation elements. What the specific 

 element of a gland shall be, or whether its secretioii contain 

 any, is dependent on the nature of its special cells; how 

 much transudation and how much specific element shall be 

 secreted at any time is controlled by the nervous system; 

 just as the contractility of a muscle depends on the endow- 

 ments of muscular tissue, and whether it shall rest or con- 

 tract and if the latter, how powerfully upon its nerve. 



