SECRETION OF SALIVA AND GASTRIC JUICE. 261 



common source, the blood, and they are poured into the alimentary canal, 

 not in a continuous flow, but intermittently as occasion may demand. The 

 epithelial cells which supply them have their periods of rest and of activ- 

 ity, and the amount and quality of the fluids which the cells secrete are 

 determined by the needs of the economy as the food passes along the canal. 

 We have now to consider how the epithelial cell manufactures its special 

 secretion out of the materials supplied to it by the blood, and how the cell 

 is called into activity by the presence of food, it may be as in the case of 

 saliva at some distance from itself, or by circumstances which do not bear 

 directly on itself. In dealing with these matters in connection with the 

 digestive juices, we shall have to enter at some length into the physiology 

 of secretion in general. 



The question which presents itself first is : By what mechanism is the 

 activity of the secreting cells brought into play ? 



While fasting, a small quantity only of saliva is poured into the mouth ; 

 the buccal cavity is just moist and nothing more. When food is taken, or 

 when any sapid or stimulating substance, or indeed a body of any kind, is 

 introduced into the mouth, a flow is induced which may be very copious. 

 Indeed the quantity secreted in ordinary life during 24 hours has been 

 roughly calculated at as much as from 1 to 2 litres. An abundant secretion 

 in the absence of food in the mouth may be called forth by an emotion, as 

 when the mouth waters at the sight of food, or by a smell, or by events oc- 

 curring in the stomach, as in some cases of nausea. Evidently in these iiir 

 stances some nervous mechanism is at work. In studying the action of this 

 nervous mechanism, it will be of advantage to confine our attention at first 

 to the submaxillary gland. 



195. The submaxillary gland is supplied with two sets of nerves. 

 These are represented in Fig. 81, which is a very diagrammatic rendering 

 of the appearances presented when the submaxillary gland is prepared for 

 an experiment in a dog, the animal being placed on its back and the 

 gland exposed from the neck. The one set, and that the more important, 

 belongs to the chorda tympani nerve (ch. </'). This is a small nerve, which 

 branches off from the facial or seventh cranial nerve in the Fallopian canal 

 before the nerve issues from the skull. Whether it really belongs to the 

 facial proper has been doubted ; in man the fibres which form it are either 

 fibres coming not from the roots of the facial proper, but from the portio 

 intermedia Wrisbergi, or, according to some, fibres which, though joining 

 the facial in the Fallopian canal, are ultimately derived from another 

 (the fifth) cranial nerve. Leaving the facial nerve, the chorda tympani 

 passes through the tympanic cavity or drum of the ear (hence the name) 

 and joins or rather runs in company (ch. /) with the lingual or gustatory 

 branch of the fifth nerve. Some of the fibres run on with the lingual right 

 down to the tongue (these are not shown in the figure), but many leave the 

 lingual as a slender nerve (ch. .), which, reaching Wharton's duct or duct 

 of the submaxillary gland (sm. d.) runs along the duct to the gland. As 

 the nerve courses along the duct nerve-cells make their appearance among 

 the fibres, and these are especially abundant just after the duct enters the 

 hilus of the gland. The fibres may be traced into the gland for some dis- 

 tance, but, as we have said, their ultimate ending has not yet been definitely 

 made out. Along its whole course up to the gland, the fibres of the chorda 

 are very fine medullated fibres, but they lose their medulla in the gland. 



The other set of nerve-fibres reaches the gland along the small arteries 

 of the gland. These are non-medullated fibres mixed with a few medullated 

 fibres, and may be traced back to the superior cervical ganglion. From 

 thence they may be traced still further back down the cervical sympathetic 



