84 



XI. The muscular parietes of the mouth are, during mastications, in 

 perpetual action. The tongue presses on the food, in every direction, 

 and brings it under the teeth ; the muscles of the cheek, especially the 

 buccinator, against which the food is pressed, force it back again under 

 the teeth, that it may be duly triturated. When the food has been suf- 

 ficiently divided, and imbued with saliva, the tip of the tongue is carried 

 to every part of the mouth, and the food is collected on its upper surface. 

 The food having been thus completely gathered together, the tongue 

 presses it against the roof of the mouth, and turning its tip upwards and 

 backwards, at the same time that its base is depressed, there is offered 

 to the food an inclined plane, over which the tongue presses it from be- 

 fore backwards, to make it clear the isthmus of the fauces, and to thrust 

 it into the oesophagus. In this course of the food along the pharynx, 

 and into the oesophagus, cosists deglutition, a function which is as- 

 sisted by the co-operation of several organs whose mechanism is rather 

 complicated. 



XII. Deglutition. In the process of deglutition, the mouth closesby the 

 approximation of both jaws ; at the same time, the submaxillary muscles, 

 the digastrici, the genio-hyoidei^ the mylo-hyoidci^ Sec. elevate the larynx 

 and pharynx, by drawing down the os hyoides together with the lower 

 jaw, which is fixed by its levator muscles. The hyoglossus muscle, at 

 the same time that it elevates the os hyodies, depresses and carries, back- 

 wards the base of the tongue*. Then the epiglottis, situated between 

 these two parts, which are brought together, is pushed downwards and 

 backwards by the base of the tongue, which lays it over the opening of 

 the larynx. The alimentary mass, pressed between the palate and the 

 upper surface of the tongue, slides on the inclined plane formed by the 

 latter, and pressed by its tip, which bends back, clears the isthmus of 

 the fauces. The mucous substance which excludes from the surface of 

 amygdalae further facilitates the passage of the food. When the food 

 has thus drooped into the pharynx, the larynx, which had risen, and had 

 come forward, and which in that motion had drawn the pharynx along 

 with it, descends and falls backwards. This last organ stimulated by the 



* In a memoir read before the French Institute, in 1813, M. Magendie related two 

 cases in which deglutition was performed without difficulty, although the epiglottis was 

 destroyed. He attributes the closure of the larynx to the action of the arytenoid mus- 

 cles, which are generally thought to be exclusively appropriated to the Voice. God- 



ration its ashes contain a considerable portion of phosphate of lime, although none of 

 that salt can be detected in it before incineration. It is this pecular substance which 

 adheres to the teeth, and gives origin to the tartar which surrounds them. This deposi- 

 tion, according to BERZELITJS, is composed of 



Earthy phosphates, 79.0 



Undecomposed mucus, 12.5 



Peculiar salivary matter, 1,0 



Animal matter soluble in mur. acid, 7.5 



100 



It cannot be doubted, that, like the other animal fluids, the constitution of this is lia- 

 ble to changes from disease. It is, however, a subject which has excited little atten- 

 tion among chemists and physiologists. BHrGsrAT.Ei.Lr found the saliva of a patient 

 labouring under an obstinate venereal disease, impregnated with oxalic acid. 



The concretions which sometimes form in the salivary ducts, chiefly consist of phos- 

 phate of lime ia coagulated albumen. Copland 



