212 EXPERIMENTAL PHYSIOLOGY 



which has been passed round it, and stimulate it by a weak interrupted 

 current — the purpose of this is to distend the duct with secretion, and 

 thus render the introduction of the cannula much easier. Open the 

 duct with a pair of sharp scissors, insert the cannula, and tie it in. 



To show variations in rate of secretion the secretion may he collected in 

 small glass tubes or capsules, having attached a short piece of rubber tailing 

 to the end of the cannula. The following plan is a very good one for class 

 demonstration. A long piece of glass tubing with thick walls and of about 

 1 mm. bore is taken. To one end of this a piece of rubber tubing is fixed, in 

 the course of which is inserted a glass T-piece, the lateral orifice being closed 

 by a piece of tubing and spring clip. The rubber tubing and a few inches of 

 the glass tubing are filled with a solution of methylene blue. The cannula is 

 then filled with fluid and connected to the long glass tube by the rubber 

 tubing. The glass tubing is then held in a horizontal position against a 

 white surface, so that the coloured column of fluid stands out clearly. It is 

 also convenient to have the tube marked with transverse lines at short 

 intervals. When the demonstration is being made to large classes, it is often 

 convenient to project an image of the coloured column on to a screen by 

 means of a lantern. As saliva is secreted the coloured fluid is moved along 

 the horizontal tube, and its rate can be very accurately watched. The 

 meniscus of the fluid can at once be brought to any position of the tube by 

 aid of the T-piece on the rubber tubing. By opening this some fluid inay be 

 'forced out or sucked in, if the end of the rubber tubing be immersed in fluid. 



We may now show the chief facts in the rate of secretion of saliva 

 by the following experiments : — 



1. Observe the rate of floiv of saliva from the unstimulated gland. 

 In animals anaesthetised with ether this rate is, as a rule, greater than 

 when other anaesthetics are employed. 



2. Stimulate the chorda. — Lift up the nerve, place it upon a pair 

 of shielded electrodes, and tetanise it with weak induced currents. The 

 rate of flow of saliva is very greatly accelerated. 



3. Stimulate the sympathetic. — The rate of flow, in the case of the 

 dog, is no greater than before stimulation. 



4. Observe the vascular changes in the gland on stimulation of the 

 two nerves. When the chorda is stimulated the gland will be dis- 

 tinctly observed to become redder, due to dilatation of its blood vessels. 

 On the other hand, sympathetic stimulation is followed by a paling of 

 the gland, due to constriction of its vessels. 



5. Take a camel's-hair pencil moistened with a - l per cent, solu- 

 tion of nicotine in 1 per cent. NaCl solution and paint the submaxillary 

 ganglion with the solution of nicotine, taking care to limit the action 

 of the nicotine to the submaxillary ganglion. Now stimulate the 

 chorda once more and a rapid flow of secretion is again obtained. In 

 the next place paint the chorda with the solution of nicotine at a spot 

 about 1^ inch from the hilum. Then once more stimulate the 

 chorda near its origin from the lingual. A free flow of saliva is 



