PRACTICAL EXERCISES 377 



under morphia. Set up an induction-machine for a tetanizing current 

 (p. 175), and connect it with fine electrodes. Fasten the dog on the 

 holder, give ether if necessary, and insert a cannula in the trachea 

 (p. 177). Then make an incision 3 or 4 inches long, through 

 skin and platysma muscle, along the inner border of the lower jaw 

 beginning about the angle of the mouth, and continuing backwards 

 towards the angle of the jaw. Such branches of the jugular vein as 

 come in the way may be generally pushed aside, but if necessary they 

 may be doubly ligated and divided. Feel for the facial artery, so 

 as to be able to avoid it. Divide the digastric muscle about its 

 anterior third, and clear it carefully from its attachments; or, without 

 dividing it, pull it outwards with a hook. The broad, thin mylo-hyoid 

 muscle will now be seen with its motor nerve lying on it. Divide 

 the muscle about its middle at right angles to its fibres, and raise it 

 carefully. The lingual nerve will be seen emerging from under the 

 ramus of the jaw. It runs transversely towards the middle line, 

 and then, bending on itself, passes forwards parallel to the larger 

 hypoglossal nerve. In its transverse course the lingual will be seen 

 to cross the ducts of the submaxillary and sublingual glands. These 

 structures having been identified, the lingual nerve is to be ligatured 

 before it enters the tongue and cut peripherally to the ligature. 

 Then a glass cannula of suitable size is to be inserted into the sub- 

 maxillary duct (the larger of the two), just as if it were a bloodvessel 

 (p. 58). A short piece of narrow rubber tubing is carefully slipped 

 on the end of the cannula. The lingual is now to be lifted by means 

 of the ligature, and traced back towards the jaw till its chorda tympani 

 branch is seen coming off and running backwards along the duct. 

 The chordo-lingual nerve (Fig. 107, p. 333) is then to be cut 

 centrally to the origin of the chorda tympani, which can now be easily 

 laid on electrodes by means of the ligature on the lingual. On 

 stimulating the chorda, the flow of saliva through the cannula will 

 be increased. The current need not be very strong. If the flow 

 stops after a short time, it can be again caused by renewed stimula- 

 tion after a brief rest. A quantity of saliva may thus be collected, 

 .and the experiments already made with human saliva repeated. 



(2) Expose the vago-sympathetic nerve in the neck on the same 

 side ; ligature it ; divide below the ligature, and note the effect 

 produced by stimulation of the upper end on the flow of saliva. 



(3) Set up another induction - machine, and connect it with 

 -electrodes. Stimulate the chorda, and note the rate of flow of the 

 saliva. Then, while the chorda is still being excited, stimulate the 

 vago-sympathetic and observe the effect. If the experiment is suc- 

 cessful, finish by stimulating the chorda for a long time. Then 

 harden both submaxillary glands in absolute alcohol, make sections, 

 stain with carmine and compare them. 



3. Effect of Drugs on the Secretion of Saliva. (i) Proceed as 

 in 2(1), but, in addition, insert a cannula into the femoral vein, and 

 while the chorda is being stimulated inject into the vein, in the 

 manner described on p. 177, TO to 15 milligrammes of sulphate of 

 atropia. This will stop the flow of saliva, and abolish the effect of 



