80 



the single or combined action of their muscles, by which the greater part 

 of the face is covered, and which may be enumerated as follows : Eleva- 

 tors of the upper lip (caninus, incisvus, leva-tores communes labiorum el my- 

 rtiformes.} Depressors of the under lip (triangularis labiomm, quadratus 

 genx.) Abductors (buccinator, zygomaticus major et minor ^ platysma 

 myoides.} Constrictors (orbicularis or is.} 



VII. The motions of the upper jaw are so confined, that some have de- 

 nied that it has any motion: it nevertheless rises a little*, when the low- 

 er jaw descends; but it is principally by the depression of the latter that 

 the mouth is opened. The muscles at the back of the neck, and that part 

 of the digastric muscle nearest the mastoid process, produce a slight ele- 

 vation of the upper jaw, which moves with the whole head, to the bones 

 of which it is firmly united. This connexion of the upper jaw with the 

 bones of the head, renders this jaw less move able in man than in the 

 greater number of animals, in which, freed from the enormous weight of 

 the skull, it stretches out in front of that cavity, over the lower jaw. As 

 we follow downwards the scale of animal existence, the motions of the 

 upper jaw is seen to increase, the further we descend from the human 

 species; it is equal to that of the lower jaw, in the reptiles, and in several 

 fishes : hence the enormous dimensions of the mouth of the crocodile and 

 shark; hence serpents frequently swallow a prey of a bulk greater than 

 their own, and would be stiffocated, but for the power they possess, of 

 suspending respiration for a long time, and of waiting patiently, till the 

 gastric juice dissolves the food, as it is swallowed. 



In the act of mastication, the upper jaw may be considered as an anvil, 

 on which the lower jaw strikes as a moveable hammer, and the motions 

 of the under jaw, the pressure it exerts, and its efforts, would soon have 

 disturbed the connexion of the different bones of which the face is formed, 

 if this unsteady edifice, merely formed of bones, in juxta position, or 

 united by sutures, were not supported, and did not transmit to the skull, 

 the double effort which presses on it from below upwards, and pushes it 

 out literally. Six vertical columns, the ascending apophyses of the supe- 

 rior maxillary bones, the orbital* processes of the malar bones, and the 

 vertical processes of the palate bones, support and transmit the effort 

 which takes place in the first direction, while the zygomatic processes 

 forcibly press the bones of the face against each other, and powerfully re- 

 sist separation outwardly or laterally. The lower jaw falls by its own 

 weight, when its elevators are relaxed; the external pterygoid muscles, 

 and those attached to the os hyoides, complete this motion, the centre of 

 which is not in the articulation of the jaw to the temporal bones, but cor- 

 responds to a line that should cross the coronoid processes, a little above 

 the angles of the jaw. It is around this axis, that, in falling, the lower 

 jaw performs a motion of rotation, by which its condyles are turned for- 

 wards, while its angles are carried backwards. In children, the coro- 

 noid processes standing off at a smaller distance from the body of the 

 bone, of which they have nearly the same direction, the centre of motion 

 is always in the glenoid cavities, which the condyles never quit, however 

 much the jaw may be depressed. By this arrangement, nature has guard- 

 ed against dislocation, which would have been frequent at an early period 



* This can only be from a motion of the whole head on the atlas ; the action of the 

 posterior belly of the digastricus is a mere assumption. Godman> 



