276 THE HUMAN BOD7. 



liquid poured out on the surfaces of the serous membranes. 

 Such secretions are known as transudata and their amount 

 is only indirectly controlled by the nervous system, through 

 the influence of the latter upon the circulation of the blood. 

 The cells lining such surfaces are not secretory tissues in 

 any true sense of the word, being merely flat, inactive, thin 

 scales protecting the surfaces. In other cases the lining 

 cells are thicker, and 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 secretion 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 either case the activity of 

 the organ is directly 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 ex- 

 cited, 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 watering 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 produc- 

 tion of these, by starting new chemical processes in the cells 

 by which a substance built up in them during rest is con- 

 verted into the specific element, which is soluble in and 

 carried off by the transudation elements. What the speci- 

 fic element of a gland shall be, or whether its secretion con- 

 tain 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 endowments of muscular tissue, and whether it shall 

 rest or contract arid if the latter how powerfully upon 

 its nerve. 



