INFLUENCE OF NERVOUS SYSTEM. 413 



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, produce, 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 production of secretion is, also, largely influenced 

 by the condition of the nervous system. It is not possible 

 to say, with certainty, whether the secretion of a gland 

 would be arrested by the division or destruction of all the 

 nerves distributed to it, for the branches of the nerves 

 are largely spread over the blood-vessels, so that their 

 destruction cannot be effected without serious injury to 

 these, and to other structures entering into the formation 

 of the gland. It is probable, however, that the influence 

 of the nerves in secretion is due mainly, if not entirely, to 

 their controlling influence on the calibre of the blood- 

 vessels. 



It appears immaterial to the perfection of secretion, 

 from which nervous centres a secreting organ receives its 



