274 SECRETION 



but when certain foreign matters are taken into the system or are injected 

 into the bloodvessels, they are eliminated by the different glandular 

 organs, both secretory and excretory. These organs seem to possess a 

 power of selection in the elimination of different substances. Thus, 

 sugar and potassium ferrocyanide are eliminated in greatest quantity by 

 the kidneys ; the salts of iron, by the kidneys and the gastric tubules ; 

 and iodin, by the salivary glands. 



The discharge of secretions is almost always accompanied with an 

 increase in the pressure of blood in the vessels supplying the glands ; 

 and it has been shown, also, that an increase in the pressure, when the 

 nerves of the glands do not exert an opposing influence, increases the 

 activity of secretion ; but this does not demonstrate that secretion is 

 due simply to an increase in the pressure of blood in the glands, although 

 this undoubtedly exerts an important influence. It is necessary that 

 other conditions should be favorable to the act of secretion for this 

 influence to be effective. Experiments have shown that pain may com- 

 pletely arrest the secretion of urine, operating undoubtedly through the 

 nervous system. When the flow of urine is arrested by pain, an increase 

 in the pressure of blood in the part fails to excite secretion. 



Influence of the Nervous System on Secretion. The fact that the 

 secretions usually are intermittent in their flow, being discharged in 

 obedience to impressions which are made only when there is a demand 

 for their physiological action, would naturally lead to the supposition 

 that they are regulated, to a great extent, through the nervous system ; 

 particularly as it is now well established that the nerves are capable of 

 modifying and regulating local circulations. This applies to a certain 

 extent to the excretions, which are also subject to considerable modi- 

 fications. 



In regard to many of the glands, it has been shown that the influ- 

 ence of the vasomotor nerves is antagonized by certain other nerves, 

 which latter are called the motor nerves of the glands. The motor 

 nerve of the submaxillary is the chorda tympani ; and as both this nerve 

 and the sympathetic, which latter contains the vasomotor filaments, 

 together with the excretory duct of the gland, can easily be exposed 

 and operated on in a living animal, many experiments have been per- 

 formed on this gland. When the parts are exposed and a tube is 

 introduced into the salivary duct, division of the sympathetic induces 

 secretion, with an increase in the circulation in the gland, the blood 

 in the vein becoming red. On the other hand, division of the chorda 

 tympani, the sympathetic being intact, arrests secretion, and the 

 venous blood coming from the gland becomes dark. If the nerves 

 are now stimulated alternately, it will be found that stimulation of the 



