150 FUNCTIONS OF NUTRITION. 



animals the non-digestion of starch by saliva has been experimentally 

 demonstrated. Colin* found the farinaceous matter of oats and starchy 

 roots recognizable by its iodine reaction, after remaining in the first 

 stomach of the ox, mixed with saliva, for twenty-four hours ; and the 

 same observer introduced into the interior of the paunch, through a 

 fistula, muslin bags containing uncooked potato starch, which were 

 found in the same cavity, still full of unaltered starch, at the end of 

 twenty and twenty-two hours. In all cases, furthermore, the saccha- 

 rine transformation of starch, as we shall hereafter see, is accomplished 

 with great energy and promptitude by other secretions in the small 

 intestine. 



It seems evident, therefore, that the sugar-producing quality of the 

 saliva is not a prominent part of its physiological action ; but that it is 

 mainly useful, by its physical properties, in facilitating mastication 

 and deglutition. 



It is also subservient, in an indirect way, to the nervous influences 

 concerned in mastication. This process is aided and controlled in 

 great measure by the sensibilities of touch and taste, in the tongue and 

 other parts of the mucous membrane. The taste notifies us of the ali- 

 mentary character of the food taken into the mouth, and its sapid 

 qualities must be fully brought out before mastication is complete. 

 Taste depends, for one of its essential conditions, on a sufficient supply 

 of saliva, since no substance can produce an impression on the gusta- 

 tory nerves unless it be fluid and capable of absorption. The saliva 

 produces this effect on the soluble ingredients of the food, such as sac- 

 charine substances, saline matters, acids, or alkalies, and brings them 

 in contact with the papillae of the tongue in sufficient quantity to 

 produce a gustatory sensation. 



The general sensibility of the tongue enables this organ to appre- 

 ciate the physical condition of the food, and its readiness for deglu- 

 tition. At the same time its muscular apparatus provides for its 

 movement in every direction. When the alimentary material is finally 

 reduced, by the saliva and mastication, to a pasty and homogeneous 

 condition, the softened mass is collected from every part of the mouth 

 by the movements of the tongue, brought together upon its upper sur- 

 face, and then pressed backward through the fauces into the pharynx 

 and oesophagus. Here it passes beyond the control of the will. It is 

 then grasped by the muscular fibres of the oesophagus, and by a con- 

 tinuous and rapid peristaltic action is carried downward into the 

 stomach. 



Gastric Juice. 



The stomach is no doubt the organ in which the most important part 

 of the digestive process is inaugurated, and which contributes most 

 largely to the chemical modification of the food. Its special secretion 



Physiologic compare des Aniinaux Domestiques. Paris, 1854, tome i., p. 603. 



