The position, then, of the duct of the parotid, which is so situated that mechanical means are brought into play, in order to insure the thorough incorporation of its saliva with the food, and the great comparative size of the gland itself, lead naturally to the in ference that the parotid is by far the most important gland in the series, the subnaaxillary and lingual having but a subordinate function. This deduction has been experimentally proved by Bernard*, who, having made an aperture at the lower part of the cesophagus of a horse, administered to the animal about sixteen ounces of oats. Fifteen or sixteen seconds after the commencement of mastication, a rounded mass made its appearance at the cesophageal opening, well triturated, perfectly moist, pasty in the interior, and covered on the exterior hy a moderately thick layer of tenacious mucus and saliva. A fresh quantity of the oats, in a similar condition, was pro jected every three-quarters of a minute. At the end of nine minutes, the mastication of the entire quantity having been finished, the ducts of the parotid were divided, so that the saliva that was secreted could be conducted out of the mouth. The same quantity of oats was again given to the animal. In this second experiment mastication did not appear to be attended with any particular incon venience, and was performed as easily as in the first. It was exerted, however, a much longer time; for a minute and a half elapsed before the first mass made its appearance at the opening : this, though well triturated, and covered on its external surface with much mucus, was considerably smaller than those masses which had escaped from the cesopha geal opening prior to the division of the ducts of the parotid. The interior of the mass, also, instead of being, like them, well mois tened and pasty, had but slight tenacity, and was comparatively dry. Mastication and deglu tition now became more and more difficult, lengthened, and laborious, so that an interval of from two minutes and a half to three minutes frequently occurred between the exit from the cesophagus of the successive masses. The horse, in its endeavours to swallow the oats which appeared to adhere to the palate, frequently gulped down a quantity of air, which escaped with noise from the cesophagus prior to the exit of the oats that had with such difficulty passed into the canal. At the end of twenty-five minutes, but little more than eleven ounces of the oats had been mas ticated and swallowed, whereas, prior to the division of the parotid ducts, sixteen ounces had been well triturated and swallowed in nine minutes. Bernard further remarks, that he collected during the second experiment the saliva that flowed from the parotid ducts, and he found that it came away in an almost continued current ; but that during the time that he administered water to the animal not a single drop of saliva escaped. The circum
stance of the smallness of the masses passed in the second experiment, and the dryness of their interior, taken together with their ex terior envelope of tenacious mucus and saliva, which was as abundant as before the division of Steno's ducts, lead to the inference that the former condition was owing to the absence of the aqueous secretion of the parotid ; the latter condition, to the fact of the submaxillary and sublingual glands being mainly engaged in the secretion of a tenacious saliva. Further experiments bring about the conclusion that the fluid from the parotid on the one hand, and from the submaxillary and sublingual on the other, are regulated by conditions special to each. Thus, the quantity of saliva secreted by the parotid of a horse is in direct ratio to the dryness of the food and the difficulty experienced in its mechanical division. The mastication of straw and hay- causes the flow of more than that of oats and farinaceous matters ; the mastication of moist forrns of food, hardly any. This, however, is by no means the case with the saliva from the sublingual and the submaxillary ducts. This always flows nearly in equal abundance whether mastication be exerted on dry or moist forms of food, and, owing to its com parative tenacity, is not easily imbibed into the centre of the masticated material, but gathers round the surface of the mass, thus favouring its passage along the alimentary canal. In a mechanical point of view, then, there are two forms of saliva : the one clear and aqueous, secreted from the parotid, and which may be denominated the " saliva of mastication," because its secretion is directly related to this act ; the other tenacious and secreted by the submaxillary and sublingual, " the saliva of deglutition," because it always lubricates the surface of the alimentary mass, whether it be submitted to mastication or not. The above views of Bernard are materially strengthened by the fact of the high develop ment of the parotid in animals that masticate, and its absence or mere rudimentary condition in those that swallow without masticating. Its comparative smallness, in relation to the submaxillary and sublingual in the human in fant, is also corroborative.
Without entering into the physiology of the secretion of the saliva, which will be found treated of elsewhere (see SALIVA, SE CRETION), it may be interestino to remark, that the salivary glands, althouglfimmediately surrounded by muscles, are not necessarily compressed in the different movements of the jaw. This conclusion has been arrived at by a series of interesting experiments and in ductions due to Borden, but into the analysis of which it would be beyond the limits of this article to enter.*