CHAPTER XXIV. 

 DIGESTION. 



The Object of Digestion. Of the various foodstuffs swal- 

 lowed, some are already in solution and ready to dialyze at 

 once into the lymphatics and blood-vessels of the alimentary 

 canal; others, such as a lump of sugar, though not dissolved 

 when put into the mouth, are readily soluble in the liquids 

 found in the alimentary canal, and need no further digestion. 

 In the case of many most important foodstuffs, however, 

 special chemical changes have to be wrought, either with the 

 object of converting insoluble bodies into soluble, or non- 

 dialyzable into dialyzable, or both. The different_secretions 

 poured into the alimentary tube act in various ways upon 

 different foodstuffs, and at last get them into a state in which 

 they can pass into the circulating medium and be carried to 

 all parts of the Body. 



The Saliva. The first solvent that the food meets with 

 is the saliva, which, as found in the mouth, is a mixture of 

 pure saliva, formed in paroticlr-subm axillary, and sublingual 

 glands, with the mucus secreted by small glands of the buccal 

 mucous membrane. This mixed saliva is a colorless, cloudy, 

 feebly alkaline liquid, "ropy" from the mucin present in 4t, 

 arid usually containing air-bubbles. Pure saliva, as obtained 

 by putting a fine tube in the duct of one of the salivary 

 glands, is more fluid and contains no imprisoned air. 



Usually but little saliva is secreted ; the presence of food 

 in the mouth, especially highly flavored or acid food, leads 

 to a more abundant flow : the mere chewing of a tasteless 

 inert substance will, however, excite some secretion. The secre- 

 tion thus brought about is reflex: the afferent fibres running 

 to the brain in the glossopharyngeal and lingual nerves, and 

 exciting there the centre from which the efferent secretory 

 nerve-fibres for the glands arise. The centre may be excited 

 in other ways: as by nausea, or through the nerves of eye or 

 nose when the sight or smell of desirable food makes " the 

 mouth water." 



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