INFLUENCE OF THE NERVOUS SYSTEM UPON SECRETION. 347 



trate this fact more fully, Bernard divided the nerves on one side, through which the 

 reflex nervous action was communicated to the kidney, leaving the other side intact. He 

 then found that increase in the arterial pressure, accompanied with pain, diminished the 

 flow of urine upon the sound side, through which the nervous action could operate, and 

 increased it upon the other. 



The influence of pressure of blood upon secretion may, then, be summed up in a few 

 words : There is always an increase in the activity of secretion when the pressure of 

 blood in the glands is increased, and a diminution when the pressure is reduced ; except 

 when there is some modifying influence operating through the nervous system. 



Influence of the Nervous System upon Secretion. The fact that the secretions are gen- 

 erally intermittent in their flow, being discharged in obedience to impressions which are 

 made only when there is a demand for the exercise of their functions, 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 modify- 

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

 excretions, which are also subject to considerable modifications. A few years ago, 

 indeed, there was considerable discussion regarding a subdivision of the reflex system of 

 nerves, which was supposed to preside over secretion and was called the excito-secre- 

 tory system. The facts which led to the description of this system of nerves had long 

 been observed, and they simply illustrated the production of the secretions in response to 

 irritation. 



Experiments have clearly demonstrated the importance of the nervous influence in the 

 production of the secretions ; but the observations of Bernard show that the effects are 

 produced mainly by increasing the activity of the circulation in the glands. This takes 

 place in, greatest part through filaments from the sympathetic system, which are dis- 

 tributed to the muscular coats of the arteries of supply. "When these filaments are 

 divided, the circulation is increased here, as in other situations, and secretion is the result ; 

 and, if the extremity of the nerve connected with the gland be galvanized, contraction 

 of the vessels follows, and the secretion is arrested. 



With regard to many of the glands, Bernard has shown that the influence of the sym- 

 pathetic is antagonized by nerves derived from the cerebro-spinal system, which latter he 

 calls the motor nerves of the glands. The motor nerve of the submaxillary is the chorda 

 tympani; and, as both this nerve and' the sympathetic, together with the excretory duct 

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

 experiments of Bernard 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 galvanized alternately, it will be found that galvanization of the sympa- 

 thetic produces contraction of the vessels of the gland and arrests secretion, while the 

 stimulus applied to the chorda tympani increases the circulation and excites secretion. 

 Enough is known of the nervous influences which modify secretion, to admit of the 

 inference that all the glands are possessed of nerves through which reflex phenomena, 

 affecting their secretions, take place. It is the motor, or functional nerve of the gland 

 through which the reflex action takes place ; the influence of the sympathetic being con- 

 stant and the same as in other parts where it is distributed to blood-vessels. 



As reflex phenomena involve the action of a nervous centre, it becomes an interesting 

 question to determine whether any particular parts of the central nervous system preside 

 over the various secretions. We must refer again to the experiments of Bernard for an 

 elucidation of this question. If a puncture be made in the space included between the 

 origin of the pneumogastrics and the auditory nerves, in the floor of the fourth ventricle, 



