44 ESSENTIiiLS OF CHEMICAL PHYSIOLOGY 



LESSON V 

 SALIVA 



1. To a little' saliva in a test-tube add acetic acid. Mucin is precipitated 

 in stringy flakes. 



2. Filter some fresh saliva to separate cells and mucus, and apply the 

 xanthoproteic or Millon's test to the filtrate ; the presence of proteid is 

 shown. 



3. Put some 0"5-per-cent. starch solution into two test-tubes. Add some 

 filtered saliva to one of them, and put both in the water-bath at 40° C. 

 After five minutes remove them and test both fluids with iodine and 

 Trommer's test. The saliva will be found to have converted the starch into 

 dextrin and sugar (maltose). 



4. The presence of potassium sulphocyanide (KCNS) in saliva may be 

 shown by the red colour given by a drop of ferric chloride. This coloiu* is 

 discharged by mercuric chloride. 



5. The reaction of saliva is alkaline to litmus paper. 



The saliva is the first digestive juice to come in contact with the 

 food : it is secreted by three pairs of salivary glands, the parotid, 

 submaxillary, and sublingual. The secretions from these differ 

 somewhat in composition, but they are mixed in the mouth, the 

 secretion of the minute mucous glands of the mouth and a certain 

 number of epithelial cells and debris being added to it. The so- 

 called ' salivary corpuscles ' are derived either from the glands them- 

 selves or from the tonsils. 



The secretion of saliva is a reflex action ; the taste or smell of 

 food excites the nerve endings of the afferent nerves (glossopharyngeal 

 and olfactory) ; the efferent or secretory nerves are contained in the 

 chorda tympani (a branch of the seventh cranial nerve) which 

 supplies the submaxillary and sublingual, and in a branch of the 

 glossopharyngeal which supplies the parotid. The sympathetic 

 branches which supply the blood vessels with constrictor nerves 

 contain in some animals secretory fibres also. 



The parotid gland is called a serous or albuminous gland ; before 

 secretion the cells of the acini are swollen out with granules ; after 

 secretion has occurred the cells shrink, owing to the granules having 

 been shed out to contribute to the secretion (see fig. 12). 



