THE DIGESTIVE JUICES 67 



are catalysing agents. That is to say, their presence induces a 

 chemical reaction to occur rapidly which in their absence also occurs, 

 but so slowly that any action at all is difficult to discover. To use 

 the technical phrase, their action is to increase the velocity of chemical 

 reactions. The enzymes are catalysts of a colloidal nature, and 

 certain properties, in which they differ from most inorganic catalysts 

 are due to this circumstance. 



THE SALIVA 



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. 15). 



The submaxillary and sublingual glands are called mucous 

 glands : their secretion contains mucin. Mucin is absent from 

 parotid saliva. The granules in the cells are larger than those of 

 the parotid gland : they are composed of mucinogen, the precursor 

 of mucin (see fig. 16). 



In a section of a mucous gland prepared in the ordinary way the 

 mucinogen granules are swollen out, and give a highly refracting 

 appearance to the mucous acini (see fig. 17). 



COMPOSITION OF SALIVA 



On microscopic examination of mixed saliva a few epithelial 

 scales from the mouth and salivary corpuscles from the tonsils 

 are seen. The liquid is transparent, slightly opalescent, of slimy 

 consistency, and may contain lumps of nearly pure mu3in. On 

 standing it becomes cloudy owing to the precipitation of calcium 

 carbonate, the carbonic acid which held it in solution as bicarbonate 

 escaping. 



Of the three forms of saliva which contribute to the mixture 

 found in the mouth, the sublingual is richest in solids (2'75 per cent.). 



F 2 



