PRACTICAL EXERCISES 375 



PRACTICAL EXERCISES ON CHAPTERS IV. AND V. 



i. Saliva. Collection and Microscopic Examination of Saliva. 

 Chew a piece of paraffin-wax, or inhale ether or the vapour of strong 

 acetic acid. The flow of saliva is increased. Collect it in a porcelain 

 capsule. Examine a drop under the microscope. It may contain a 

 few flat epithelial scales from the mouth and a few round granular 

 bodies, the salivary corpuscles, the granules in which often show a 

 lively, dancing movement (Brownian motion). Filter the saliva to 

 free it from air bubbles, and perform the following experiments : 



(a] Test the reaction with litmus paper. It is usually alkaline. 

 An acid reaction may indicate that bacterial processes are abnormally 

 active in the mouth. 



(b] Add dilute acetic acid. A precipitate indicates the presence 

 of mucin (p. 296). Filter. 



(c] Add a drop or two of silver nitrate solution. A precipitate 

 insoluble in nitric acid, soluble in ammonia, proves that chlorides 

 are present. 



(d] Add to another portion a few drops of dilute ferric chloride, 

 and the same quantity to as much distilled water in a control test- 

 tube. A reddish coloration is obtained, due to the presence of sulpho- 

 cyanic acid, which is combined with potassium and other bases in 

 the saliva. The colour is discharged by mercuric chloride. Or, a 

 drop of saliva may be allowed to fall from the mouth on a test paper 

 (prepared by soaking filter paper with a dilute starch solution, con- 

 taining a little iodic acid, and allowing it to dry in the air). The 

 paper is coloured blue by the union of the starch with iodine set free 

 from the iodic acid by the action of the sulphocyanic acid. 



(e] Take some boiled starch mucilage, and test it for reducing 

 sugar by Trommer's test (p. 23). If no sugar is found, take three 

 test-tubes, label them A, B, and C, and nearly half fill each with the 

 boiled starch. To A add a little saliva/ to B some saliva which has 

 been boiled, to C a little saliva which has been neutralized, and as 

 much 0*4 per cent, hydrochloric acid as has been taken of the mucil- 

 age, so as to make the strength of the acid in the mixture o'2 per 

 cent, or the same as that of the gastric juice. Put the test-tubes into 

 a water-bath at about 40 C. In a few minutes test the contents for 

 reducing sugar. Abundance will be found in A, none in B nor in C. 

 In B the ferment ptyalin has been destroyed by boiling ; in C its 

 action has been inhibited by the acid. If the test-tubes have been 

 left long enough in the bath, no blue colour will be given by A on 

 the addition of iodine, but a strong blue colour by B and C ; /.*., the 

 starch will have completely disappeared from A. 



(/) Put some starch in a test-tube, add a little saliva, and hold in 

 the hand or place in a bath at 40 C. On a porcelain slab place 

 several separate drops of dilute iodine solution. With a glass rod 



* As it filters slowly, unfiltered saliva may be used for Experiments. 



