CHAMBERS'S INFORMATION FOR THE PEOPLE. 



turn with disgust from a mess consisting of these 

 constituents, even in proper proportions, if it were 

 not properly cooked. The best example of a 

 natural food is milk. It contains water, albumen 

 in the form of caseine or cheese, fat in the form 

 of butter, sugar, and various salts. Hence it is 

 nature's food for all young animals of the mam- 

 malian group. 



A daily amount of food varying from 33 to 35 

 ounces of dry food is sufficient to maintain the 

 health of an adult man, not engaged in active 

 labour, and of this a fourth or fifth part should be 

 animal food ; but in special cases much more or 

 less may be taken with advantage. The arrange- 

 ment and form of the teeth in man indicate that 

 he should live on a mixed diet of animal and 

 vegetable food ; though, no doubt, there are ex- 

 amples on record of individuals who have lived in 

 good health for many years entirely on either 

 animal or vegetable food. By mixing these in 

 proper portions, we are nourished on a smaller 

 bulk of food than if we lived on either one or the 

 other. 



2. MASTICATION. 



Mastication is effected in the cavity of the 

 mouth by means of the teeth, which fit into sockets 

 in the upper and lower jaw-bones. The upper jaw 

 is immovable, or only movable with the entire 

 head ; but the lower jaw, with its teeth, is capable 

 of moving upwards, downwards, backwards, for- 

 wards, and laterally, by means of the powerful 

 muscles of mastication. It is by the varied move- 

 ments of the lower teeth against the upper, through 

 the action of these muscles, that food is broken 

 down or masticated. In the adult there are 32 

 teeth, 1 6 in each jaw, and 8 on each side. There 

 are from before backwards, beginning in the 

 middle line of the jaw, 2 incisors or cutting 

 teeth on each side ; i canine or eye-tooth, for 

 seizing ; 2 premolars or bicuspids, for tearing ; 

 and 3 molars or grinders, for crushing and break- 

 ing up the food. The body and greater bulk of 

 each tooth consists of a substance called dentine, 

 composed of branching tubes ; the top or crown is 

 covered by a cap of enamel, a very hard substance, 

 made of small hexagonal prisms ; and the fang or 

 root is protected by a layer of a material resembling 

 bone, called crusta petrosa, or cement. In the 

 centre of each tooth there is a cavity containing 

 a pulpy matter, in which are nerves and blood- 

 vessels. 



3. INSALIVATION. 



Insalivation is effected by the admixture of the 

 secretions of the three pairs of salivary glands (the 

 parotids, the sub-maxillaries, and the sub-linguals), 

 and of the mucus secreted by numerous small 

 glands beneath the lining of the cheeks, gums, and 

 tongue, called buccal glands, with the triturated 

 food. The common saliva formed by the com- 

 bined secretion of these various secreting organs, 

 is a colourless, slightly turbid, viscid, inodorous, 

 and tasteless fluid. In the normal state, its reac- 

 tion is alkaline. Saliva does not contain more 

 than five or six parts of solid constituents to 

 995 or 994 -parts of water. The daily quantity 

 of saliva secreted by an adult man is estimated 

 at about 48 ounces, but the activity of the 

 in 



salivary glands is dependent upon various in- 

 fluences and conditions. Thus, movement of 

 the lower jaw, as in masticating, speaking, or 

 singing, increases the secretion acrid and 

 aromatic substances and hard dry food also 

 increase it. It is also under the influence of 

 mental emotions and desires, through the nervous 

 system, for the sight of a feast or tempting dish 

 may make one's ' mouth water.' 



The uses of the saliva in reference to digestion 

 are partly mechanical and partly chemical. The 

 chemical use of the saliva is to convert the starchy 

 portions of the food into grape-sugar, and thus 

 to promote its absorption. It also assists in 

 speech and swallowing. The public speaker can- 

 not articulate when his mouth becomes dry, and 

 we cannot swallow a perfectly dry powder. 



4. DEGLUTITION. 



Deglutition is the act by which the food is 

 transferred from the mouth to the stomach. The 

 mouth leads into a cavity called the pharynx. 

 Between it and the mouth is the pendulous or soft 

 palate, which is a movable muscular partition 

 that separates the two cavities during mastication. 

 As soon, however, as the latter act is accomplished, 



Fig. I. Human Alimentary Canal : 



a, oesophagus : b, stomach ; c, cardiac orifice ; d, pylorus ; e, small 

 intestine ; f, biliary duct ; g, pancreatic duct ; h, ascending 

 colon ; i, transverse colon ; /, descending colon ; k, rectum. 



and the bolus is pressed backwards by the tongue, 

 the soft palate is drawn upwards and backwards, 

 so as to prevent the food passing into the nose. 

 The opening of the windpipe is closed by a lid 

 called the epiglottis. The bolus or pellet of food 

 having arrived near the oesophagus or gullet 

 (which is continuous inferiorly and posteriorly 



