SECRETION. ?>'27 



becomes florid when, on the introduction of food, its glands begin to 

 secrete; the mammary gland becomes much more vascular during lacta- 

 tion; and all circumstances which give rise to an increase in the quantity 

 .of material secreted by an organ produce, coincidently, an increased sup- 

 -ply of blood; but we have seen that a discharge of saliva may occur under 

 extraordinary circumstances, without increase of blood-supply (p. 233), 

 and so it may be inferred that this condition of increased blood-supply 

 is not absolutely essential. 



(2.) When the blood contains more than usual of the materials which 

 the glands are designed to separate or elaborate. Thus, when an excess 

 of nitrogenous waste is in the blood, whether from excessive exercise, or 

 from destruction of one kidney, a healthy kidney will excrete more urea 

 than it did before. 



(3.) Influence of the Nervous System on Secretion. The process of 

 secretion is largely influenced by the condition of the nervous system. 

 The exact mode in which the influence is exhibited must still be regarded 

 as somewhat obscure. In part, it exerts its influence by increasing or 

 diminishing the quantity of blood supplied to the secreting gland, in vir- 

 tue of the Jiower which it exercises over the contractility of the smaller 

 blood-vessels; while it also has a more direct influence, as was demon- 

 strated at length in the case of the submaxillary gland, upon the secreting 

 cells themselves; this may be called trophic influence. Its influence over 

 secretion, as well as over other functions of the body, may be excited by 

 causes acting directly upon the nervous centres, upon the nerves going 

 to the secreting organ, or upon the nerves of other parts. In the latter 

 case, a reflex action is produced: thus the impression produced upon the 

 nervous centres by the contact of food in the mouth, is reflected upon 

 the nerves supplying the salivary glands, and produces, through these, a 

 more abundant secretion of saliva (p. 232). 



Through the nerves, various conditions of the brain also influence the 

 secretions. Thus, the thought of food may be sufficient to excite an 

 abundant flow of saliva. And, probably, it is the mental state which ex- 

 cites the abundant secretion of urine in hysterical paroxysms, as well as 

 the perspirations and, occasionally, diarrhoea, which ensue under the influ- 

 ence of terror, and the tears excited by sorrow or excess of joy. The qual- 

 ity of a secretion may also be affected by the mind; as in the cases in which, 

 through grief or passion, the secretion of milk is altered, and is sometimes 

 so changed as to produce irritation in the alimentary canal of the child, 

 or even death (Carpenter). 



Relations between the Secretions. The secretions of some of 

 the glands seem to bear a certain relation or antagonism to each other, 

 by which an increased activity of one is usually followed by diminished 

 activity of one oT more of the others; and a deranged condition of one is 

 apt to entail a disordered state in the others. Such relations appear to 



