408 SECRETION. 



materials for any secretion that it may contain, and in the 

 conditions of the nerves of the glands. 



In general, an increase in the quantity of blood traversing 

 a gland, coincides with an augmentation of its secretion. 

 Thus, the mucous membrane of the stomach becomes florid 

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

 secrete : the mammary gland becomes much more vascular 

 during lactation; and it appears that all circumstances 

 which give rise to an increase in the quantity of material 

 secreted by an organ, produced, coincidently, an increased 

 supply of blood. In most cases, the increased supply of 

 blood rather follows than precedes the increase of secre- 

 tion ; as, in the nutritive processes, the increased nutrition 

 of a part just precedes and determines the increased 

 supply of blood ; but, as also in the nutritive process, an 

 increased supply of blood may have, for a consequence, an 

 increased secretion from the glands to which it is sent. 



Glands also secrete with increased activity when the 

 blood contains more than usual of the materials they are 

 designed to separate. Thus, when an excess of urea is 

 in the blood, whether from excessive exercise, or from 

 destruction of one kidney, a healthy kidney will excrete 

 more than it did before. It will, at the same time, grow 

 larger : an interesting fact, as proving both that secretion 

 and nutrition in glands are identical, and that the presence 

 of certain materials in the blood may lead to the formation 

 of structures in which they may be incorporated. 



The process of secretion is, also, largely influenced by 

 the condition of the nervous system. 



The exact mode in which the nervous system influences 

 secretion must be still 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 in- 

 fluence analogous to the trophic influence referred to in the 



