Insalivation.—There is no separate or super added process for insalivation; it proceeds con temporaneously with mastication, the motions which are necessary for the one supplying equally the required conditions of the other ; while the food is being comminuted by the teeth, dispersed by their action to different parts of the mouth, recollected by the tongue to be again dispersed, and so on, the salivary secretion is freely mixed with it, and reduces it to a homogeneous pulp : thus mastication facilitates insalivation by breaking up the food, and insalivation facilitates mastication by softening it. As one set of acts performs the two processes, of course there are not any additional movements of the tongue to describe.
Deglutition.—When the food has attained a sufficient moisture and softness, which is ap preciated by the tongue's sense of touch, it is collected into a mass, and the process of deglutition commences. Physiologists have divided this process, and the division is a good one, into three stages, which may be distin guished respectively as the oral, the pharyngeal, and the oesophageal: the first conducts it past the anterior pillars of the fauces, the second in cludes its transmission from that point through the pharynx into the esophagus, and the third commences with its arrival at the oeso phagus, and terminates with its entrance into the stomach. The first is entirely voluntary; the second is of a mixed nature, engaging partly voluntary and partly involuntary muscles, and, though practicable at will, is yet impres sible on the transference to the back part of the tongue of the material to be swallowed ; the third is wholly involuntary. With the two first alone the tongue is engaged, and, there fore, of these alone I shall speak.
The first stage is merely the reference of the ball of alimentary matter to a point on the back of the tongue, posterior to the anterior pillars of the Sauces. This is effected by the pressure of the tongue against the palate, whereby the food is forced back between the two.
The second stage is a much more complex process, involving a more varied mechanism, and engaging in it different parts, the tongue, the pillars of the fauces, the soft palate, the larynx, and all the muscles of the pharynx. As soon as the food has passed the anterior palatine arch, that arch contracts, and by its constriction entirely prevents the regression of the food into the mouth ; at the same time the base of the tongue, and with it the food, is carried further back, and a second closure takes place from the approximation of the posterior pillars of the fauces, produced by the contraction of the palato-pharyngeal muscles that form them: this part of the mechanism requires a little explanation. The contraction of the anterior pillars of the fauces closes the entrance into the mouth by a constriction or sphincter-like action: this is due to the general circular form of the constrictor isthmi fau cium occasioned by the inward curvature of the upper and lower extremities of each potato glosses muscle. The contraction of the palato
pharyngei is not of this nature; they have not the same inward curvature above and below, and their inferior attachments to the pos terior borders of the thyroid cartilage are capable of very little approximation ; when, therefore, they contract, the soft palate being fixed, they approach one another laterally like two curtains, leaving a narrow chink in the middle, wider below than above. As soon as the food has passed this point, this contraction takes place, so that the two muscles of the opposite sides almost touch, the chink between them being occupied by the relaxed uvula ; the passage into the posterior nares and upper part of the pharynx is thus cut off, which has induced Dzondi to call the posterior palatine arch the velum palati posteriu.s. At the same time that this is taking place, the base of the tongue is thrown back upon the epiglottis*,the larynx being drawn upwards and forwards to meet it, so that the rima glottidis is com pletely closed, and the food glides safely down, over the inclined plane thus formed, into the pharynx raised and dilated to receive it : the food then comes within the grip of the constrictors of the pharynx, which successively pass it downwards to the msophagus. This process takes place so rapidly that it is difficult to trace its parts in succession, and indeed some of them, which apparently succeed one another, are in reality contemporaneous : thus, the first stage—the raising of the dorsum of the tongue to the palate—is that which mainly contributes to the inclined plane of the second stage; and the raising and carrying forwards of the larynx under the tongue is that which principally dilates the pharynx:I The third stage, or the cesophageal, conducts the food to the stomach ; it is of that peri staltic or vermicular nature that characterises all the succeeding movements of the alimentary canal ; the muscles concerned are entirely involuntary, and the nature of the act purely reflex. The tongue is not concerned in it.
Speech. — The tongue is the instrument principally engaged in those modifications of the oral passages which give rise to articulate sounds, which, definitely grouped and com bined by man, and taken as the representa tives of certain objects, actions, qualities, and relations, constitute Language. The con sideration of this interesting subject, however, will more appropriately fall under the article VOICE, to which the reader is referred.