266 OF FOOD AND THE DIGESTIVE PROCESS. 



the earthy phosphates, that the tartar which collects about the teeth is 

 chiefly composed, the particles of these being held together by about 

 20 per cent, of animal matter ; and the composition of the concretions, 

 which occasionally obstruct the salivary ducts, is nearly the same. 



467. The quantity of Saliva formed during the twenty-four hours, 

 has been estimated at from 15 to 20 ounces ; but on this point it is im- 

 possible to speak with certainty. The secretion is by no means con- 

 stantly flowing ; indeed it is almost entirely suspended, when the 

 masticator muscles and tongue are at perfect rest, unless it be excited 

 by any mental cause ; and hence it is, that . the mouth becomes dry 

 during sleep, if it be not kept closed. The flow of Saliva takes place 

 just when it is most wanted ; that is, when food has been taken into the 

 mouth, and when the operation of mastication is going on. But it will 

 also take place, -especially in a hungry person, at the sight, or even at 

 the idea, of savoury food ; as is implied by the common expression of 

 the "mouthwatering" for such an object. The influence thus exercised 

 over it by tbe emotional state of the mind, is probably conveyed to the 

 salivary glands by the Fifth pair ; which contains many of the gray or 

 organic filaments, and which seems to take the place, in the Head, of a 

 distinct sympathetic system. 



468. Having been conveyed into the stomach, the food is submitted 

 to the action of the Gastric fluid, which is secreted in the walls of that 

 organ. This fluid is not present in the empty stomach ; its secretion 

 being excited by the presence of food, or by the irritation of the walls 

 of the organ by some solid body. In the intervals between the diges- 

 tive process, the mucous membrane is of a light pink hue ; but it 

 becomes more turgid with blood, when the presence of food calls for 

 the activity of its secreting processes. It is of a soft and velvet-like 

 appearance ; and it is constantly covered with a very thin transparent vis- 

 cid mucus, which has neither acid nor alkaline reaction. By applying 

 aliment or other stimulants to the internal coat of the stomach, and by 

 observing the effect through a magnifying glass, numerous minute pa- 

 pillae can be seen to erect themselves upon the mucous membrane, so as 

 to rise through the coating of mucus ; and from these is poured forth a 

 pure, limpid, colourless, slightly viscid fluid, having a distinctly acid 

 reaction, which is the Gastric juice. This fluid is secreted by follicles, 

 which are lodged in the walls of the stomach, and which closely resemble 

 those that elsewhere secrete mucus ; but they are usually of more 

 complex structure, and are more numerous. 



469. If the Mucous membrane of the stomach be divided by a section 

 perpendicular to its walls, it is seen to be made up, as it were, of tubu- 

 lar follicles closely applied to each other ; their blind extremities resting 

 upon the submucous tissue, and their open ends being directed towards 

 the cavity of the stomach. In some situations these tubuli are short 

 and straight ; in other parts they are longer, and present an appearance 

 of irregular dilatation or partial convolution (Fig. 80, 1). This is their 

 usual character, especially near the cardiac orifice of the stomach ; but 

 near the pyloric orifice they have a much more complex structure (Fig. 

 80, 2). These tubular follicles are arranged in bundles or groups, and 

 are surrounded and bound together by a fine areolar membrane ; and 



