450 DIGESTION AND ABSORPTION 



still show a distinct blue colour, while those at the other end of the 

 series will be brown or yellow, and the intermediate tubes bluish-violet. 

 Suppose D is the last tube still showing a bluish tint, then in the next 

 higher tube, C, all the starch has been hydrolysed at least to dextria 

 that is, 1-3 c.c. of the ten-times diluted saliva, or 0-13 of the original 

 saliva, has been sufficient to change all the starch in 10 c.c. of the I per 

 cent, solution. With another specimen of saliva the same result might 

 be reached in tube E, containing an amount of ferment equal to that 

 in 0-06 c.c. of the original saliva. We could then conclude that the 

 diastatic power of the second saliva was about twice as great as that 

 of the first. A closer approximation can now be made by setting up 

 two fresh tubes (C and E respectively for the two salivas) and deter- 

 mining the time required for the blue reaction with iodine to disappear, 

 taking out a drop from time to time and testing on a porcelain slab. 



(i) Put a little distilled water into a porcelain capsule, and bring the 

 water to the boil. Now put into the mouth some boiled starch paste, 

 and move it about as in mastication. After half a minute spit the 

 starch out into the boiling water. Divide the water into two portions. 

 Test one for sugar, and the other for starch. Repeat the experiment, 

 but keep the starch two minutes in the mouth. Report the result. 



(/) Starch solution to which saliva has been added is placed in a 

 dialyser tube of parchment-paper for twenty-four hours. At the end 

 of that time the dialysate (the surrounding water) should be tested for 

 sugar and for starch. Sugar will probably be found, but no starch. 

 If no reaction for- sugar is obtained, the dialysate should be concen- 

 trated on the water-bath, and again tested. 



6. Stimulation of the Chorda Tympani. (i) Having previously 

 studied the anatomy of the mouth and submaxillary region in the dog 

 by dissecting a dead animal (Fig. 174), put a good-sized dog under 

 morphine. Set up an induction -machine for a tetanizing current 

 (p. 198), and connect it with fine electrodes. Fasten the dog on the 

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

 (p. 199). 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 

 li gated 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 Uhe 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 submaxillary duct (the larger of the two), just as if 

 it were a bloodvessel (p. 63). 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. 160, p. 386) is then to be 

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



