1152 



TONGUE. 



of the middle fibres of the genio-glossus, which 

 depress its centre, a longitudinal channel, 

 which receives the nipple, and transmits the 

 milk to the pharynx as long as a vacuum con- 

 tinues to be formed. In drinking liquids from 

 a vessel the tongue forms a channel for its 

 transmission, but it flows down to the pharynx 

 by its own proper gravity. 



Mastication. As far as relates to the tongue, 

 which here, however, is only subsidiary to the 

 teeth, the mechanism of mastication may be 

 divided into three stages: first, that of 

 placing the food in an advantageous position 

 with regard to the teeth; secondly, affecting 

 the position of the food in a definite manner 

 when under the action of the teeth ; thirdly, 

 collecting the scattered portions of masticated 

 aliment prior to deglutition. Immediately on 

 the introduction of a morsel of food into the 

 mouth, either bitten by the incisors or other- 

 wise, it is at once transferred to the molars, 

 so that it shall project beyond them, outwards, 

 against the cheek ; the cheek is then pushed 

 against it by the action of the buccinator, and 

 the food is slowly driven across the teeth, 

 which are rhythmically opened and closed, the 

 tongue at the same time pushing moderately 

 against it on the inside and so regulating the 

 movement imparted to it by the cheek. Thus 

 we see that the food under mastication is sub- 

 jected to an equable and regulated motion ; 

 that it is placed, as it were, between two 

 movable walls, and that by the even lateral 

 movements of these walls, and the rhythmical 

 vertical action of the teeth, its perfect masti- 

 cation is secured. As soon as the cheek has 

 pushed it inwards as far as it can, an interval 

 in the rhythmical closure of the teeth takes 

 place, and the tongue restores it to its former 

 position, again to be pushed inwards, and so 

 on. The equable mastication of the food is 

 secured much in the same way as the even 

 motion of a rod of timber, under the blade of 

 a circular saw, secures the cutting off* of pieces 

 of equal thickness. Any one who watches 

 himself whilst eating will at once observe the 

 sets of rhythmical action, interrupted by short 

 interval, in which the food is restored to its 

 necessary position between the teeth. The 

 third stage, that of collecting the food from 

 all parts "of the mouth, admits of no particular 

 description. 



Insalivation. There is no separate or super- 

 added process forinsalivation; 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 cesophageal: 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 oesophagus, 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 fauces. 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 palato- 

 glossus 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 



