334 HANDBOOK OF PHYSIOLOGY. 



ducts to u considerable distance within the substance of the glands whose 

 secretions can be rapidly expelled. Saliva and milk, for instance, are 

 sometimes ejected with much force. 



Circumstances Influencing Secretion. The principal conditions 

 which influence secretion are (1) variations in the quantity of blood, (2) 

 variations in the quantity of the peculiar materials for any secretion that 

 the blood may contain, and (3) variations in the condition of the nerves 

 of the glands. 



(1.) An increase in the quantity of blood traversing a gland, as in 

 nearly all the instances before quoted, coincides generally with an aug- 

 mentation of its secretion. Thus, the mucous membrane of the stomach 

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

 crete; the mammary gland becomes much more vascular during lactation; 

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

 material secreted by an organ produce, coincidently, an increased supply 

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

 extraordinary circumstances, without increase of blood-supply, and so it 

 may be inferred that this condition of increased blood-supply is not abso- 

 lutely essential. 



(2.) An increase %n the amount of the materials which the glands are 

 designed to separate or elaborate, contained in the Uood supplied to them, 

 increases the amount of any secretion. Thus, when an excess of nitro- 

 genous 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 

 virtue of the power which it exercises over the contractility of the smaller 

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

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

 themselves; this may be called trophic influence. Its influence over se- 

 cretion, 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 the saliva. 



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 



