312 SECRETION. 



activity of secretion. The experiments of Bernard on this point show the 

 influence of pressure upon the salivary and renal secretions, particularly the 

 latter. After inserting a tube into one of the ureters of a living animal, so 

 that the activity of the renal secretion could be accurately observed, the 

 pressure in the renal artery was increased by tying the crural and the 

 brachial. It was then found that the flow of urine was markedly increased. 

 The pressure was afterward diminished by the abstraction of blood, which 

 was followed by a corresponding diminution in the quantity of urine. The 

 same phenomena were observed in analogous experiments upon the submax- 

 illary secretion. These facts, however, do 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 every 

 condition should be favorable to the act of secretion for this influence to be 

 effective. Experiments have shown that pain may completely arrest the 

 secretion of urine, operating undoubtedly through the nervous system. If 

 the flow of urine be arrested by pain, an increase in the pressure of blood in 

 the part fails to excite the secretion. 



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

 tions are generally intermittent in their flow, being discharged in obedi- 

 ence 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 regulat- 

 ing local circulations. The same facts apply, to a certain extent, to the 

 excretions, which are also subject to considerable modifications. 



It is evident that the nervous system has an important influence in the 

 production of the secretions ; and this is exerted largely through modifica- 

 tions in the activity of the circulation in the glands. This takes place in 

 greatest part through vaso-motor nerves distributed to the muscular coats of 

 the arteries of supply. When these nerves are divided, the circulation is in- 

 creased here, as in other situations, and secretion is the result ; and if the 

 extremity of the nerve connected with the gland be stimulated, contraction 

 of the vessels follows, and the secretion is arrested. 



With regard to many of the glands, it has been shown that the influence 

 of the vaso-motor nerves is antagonized by certain other nerves, which latter 

 are called the motor nerves of the glands. The motor nerve of the submax- 

 illary is the chorda tympani ; and as both this nerve and the sympathetic, 

 which latter contains the vaso-motor filaments, together with the excretory 

 duct of the gland, can be easily exposed and operated upon in a living animal, 

 many experiments have been performed upon this gland. When all these 

 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 be now 

 stimulated alternately, it will be found that stimulation of the sympathetic 



