306 HANDBOOK OF PHYSIOLOGY. 



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

 garded 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 is 

 described 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 impres- 

 sion 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 

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

 as the perspirations, and occasionally diarrhoea, which ensue under the 

 influence of terror, and the tears excited by sorrow or excess of joy. 

 The quality of a secretion may also be affected by mental conditions, 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. 



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 or more of the others; and a deranged condition of one is apt to 

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

 among the various mucous membranes; and the close relation between 

 the secretion of the kidney and that of the skin is a subject of constant 

 observation. 



The Mammary Glands. 



Structure. The mammary glands are composed of large divisions or 

 lobes, and these are again divisible into lobules the lobules being com- 

 posed of the convoluted and dilated subdivisions of the main ducts 

 (alveoli) held together by connective tissue. The lobes and lobules too 

 are bound together by areolar tissue; penetrating between the lobes and 

 covering the general surface of the gland, with the exception of the 

 nipple, is a considerable quantity of yellow fat, itself lobulated by 

 sheaths and processes of tough areolar tissue (fig. 221) connected both 

 with the skin in front and the gland behind ; the same bond of connec- 



