Of the Salivary Glands.

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-so ample is the provision for moving this organ to different parts of the fauces, whether to bruise the softer parts of the aliment against the palate, to mix it with the saliva, to place it under the pressure of the teeth, to assist in determining the taste and other sensible qualities of bodies in contact with the finely organized papillæ on its mucous surface, to urge the masticated food towards the pharynx, or to give articulation to vocal sounds.

The saliva is a limpid fluid like water, but much more viscid: it has neither smell nor taste: its specific gravity according to Dr. Thomson is 1.0038. Its constituents according to Berzelius are as follows:

The salivary glands are of an ochrey colour, and are composed of numerous molecules of different sizes, connected together by a very firm cellular texture.

The parotid gland is the largest of the three: it occupies the hollow between the mastoïd process of the temporal bone and the branch of the lower jaw; its duct, which is commonly termed the Stenonian duct, passes across the masseter muscle to perforate the buccinator and open upon the membrane of the fauces opposite to the middle molaris of the upper jaw. A small gland termed the socia parotidis adheres to the Stenonian duct in its passage over the masseter. The portio dura traverses

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130

Quantity of Saliva formed at each Repast.

the substance of the parotid gland, which appears to derive nerves from this source, from the superficial temporal branch of the third division of the fifth, and from the second cervical nerve.

The submaxillary gland is of an oval form: it is placed above the digastricus between the lower jaw and the mylohyoïdeus muscle: its duct, termed the duct of Whar ton, opens upon the mucous surface of the fauces at the side of the frænum linguæ. The nerves of this gland are branches from the gustatory upon which a ganglion is formed.

The sublingual gland is frequently continuous with the posterior portion of the submaxillary gland, is oblong, and placed between the mylohyoïdeus and the membrane of the mouth; its principal ducts or duct open into the duct of Wharton; several smaller ducts. open from it into the furrow by the side of the tongue : its nerves are derived from the gustatory.

In the case described by Dr. Gairdner, to which I have already referred, from six to eight ounces of saliva were observed to be discharged during a meal, which consisted of broth injected through the divided œsophagus into the stomach: the quantity is probably greater which is produced under the stimulus of ordinary mastication. With this fluid the food is mixed while undergoing comminution in the fauces. The food acquires at the same time the temperature of the body.

It does not appear that any notable effect is produced upon the aliment through the conjoined influence of the saliva and an elevated temperature. M. Krimer held in his mouth a slice of ham weighing a drachm for three hours. At the expiration of this time the morsel had become

Commencement of Deglutition.

131

white upon its surface, and had gained twelve grains in weight". Perhaps the qualities of the saliva are simply calculated to produce a ready mixture of the various kinds of food triturated with it in the mouth.

The nature of sensations of taste will be afterwards described. The gratification of the sense of taste seems but to whet the appetite of hunger. To allay hunger, deglutition takes place instinctively we swallow the morsel thrown by the tongue to the back part of the fauces, as soon as its sapid juices have been diffused through the mouth, and its diminished resistance shows it to have attained a state fit for deglutition :-a capacious sac termed the pharynx receives it.

The pharynx opens at the fore part, into the cavities of the nostrils, into the fauces, into the larynx: it is lined by a prolongation of the mucous surface common to these three parts: it is suspended to the basilar process of the occipital bone, and attached laterally to the pterygoid processes and cornua of the os hyoïdes; thence it becomes narrower to the first ring of the trachea, where the alimentary canal assumes a cylindrical form, and receives the name of œsophagus.

The muscles which raise the os hyoïdes raise the pharynx with it: at the commencement of deglutition all the parts of the throat visibly ascend: the pharynx is drawn upwards to receive the morsel thrust towards it by the pressure of the tongue: and one muscle, the stylo-pharyngeus, which concurs in producing this movement, seems specially intended in addition to expand the pharynx. Three muscles throw their fibres round the pharynx,

Versuch einer Physiologie des Blutes. Leipsig, 1820.

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Use of the soft Palate.

which are termed its upper, middle, and lower constrictors their action is such as to compress any substance that has found entrance into the pharynx, and thus to expel it. But the pharynx is open towards several passages, and the contrivances are remarkable, and well deserve attention, which limit the progress of the food to one direction only, and force it to descend towards the oesophagus, instead of making its escape by the nostrils, the fauces, or the larynx.

What is termed the soft palate is a flap of flexible elastic substance about of an inch in thickness and an inch in depth, which hangs as a loose curtain from the posterior edge of the palatine plate of the palate bones. The centre of its unattached margin is prolonged to form the uvula. Laterally two crescentic folds of mucous membrane are reflected from the soft palate to the sides of the tongue and pharynx: between these arches the tonsil gland is placed on either side, which secretes a viscid mucus, that becomes yellow and ropy upon slight inflammation. Each crescentic fold contains muscular fibres; the anterior contains the constrictor isthmi faucium; the posterior, the palato-pharyngeus: these muscles depress the soft palate, and narrow the aperture leading from the fauces.

By these means the communication with the fauces is so straitened, that the pressure of the tongue readily precludes the return of the food into the mouth when the constrictor superior pharyngis contracts.

But the principal office of the soft palate in deglutition consists in protecting the posterior openings of the nostrils for this purpose it is necessary not merely that this flap of yielding flesh be carried before the food to the

The Aperture of the Larynx how protected. 133

back of the pharynx; but that adequate tension be given to it to afford a competent resistance against the pressure of the constrictor superior pharyngis and of the tongue upon the morsel swallowed. Two muscles are provided in addition to those already described to give the tension required to the soft palate, namely, the levator palati mollis and the circumflexus palati: both descend obliquely forwards, the cartilaginous part of the Eustachian tube being interposed between them, from the extremity of the petrous portion of the os temporis: but while the former is directly expanded into the substance of the soft palate, the latter is previously reflected round the hamular process of the sphenoïd bone, and reascends to its insertion: the two muscles thus become opposed in their action, and drawing in different ways upon the soft palate, contribute to extend it over the pharynx behind the opening into the nostrils. Something of the effect of these parts in deglutition may be seen on pressing down the tongue with the handle of a spoon, and conveying the instrument towards the root of the tongue and the tonsils, when the peculiar sensibility of the back part of the fauces is excited, and an instinctive and irresistible action of deglutition ensues.

The mode in which the respiratory tube is protected under these circumstances can only be well understood in connexion with the anatomy of the larynx. The epiglottis, a thin portion of elastic cartilage, rises vertically at the root of the tongue, and is broad enough when carried backwards to cover the aperture of the glottis : and there is no doubt that in ordinary deglutition this cartilage is pressed down by the food upon the orifice of the larynx. But the epiglottis was removed from ani