496 SPECIAL PHYSIOLOGY. 



I I 



I 5-7 ,1 



C., x 7-9- x/ .,C 



B. X 9-12 .,B 



B.,. \8-10/ ., B 



M,. -10-12 ,...M 



M-,... 5-7 .....M 



M\.J - 12-14- ...... - M 



-17-25 



In accordance with the increased number and size of the permanent 

 teeth, contemporaneous alterations take place in both jaws. In youth, 

 the alveolar border is almost semicircular, but in the adult, semi- 

 elliptical ; it is, of course, shallow in the child, and deeper and broader 

 in the adult ; its hinder part especially, enlarges for the accommoda- 

 tion of the permanent molars. At first, the wisdom teeth of the upper 

 jaw, lie behind and above the second molars ; in the lower jaw, these 

 teeth are embedded in the base of the coronoid processes, but descend 

 to their proper position, as the jaw elongates. In the infant, the 

 angle formed behind by the lower jaw is very obtuse ; in the adult, it 

 is nearly a right angle ; but in old age, when the teeth have fallen 

 out, it again becomes more obtuse. The obtuseness of this angle favors 

 the approximation of the edges of the jaws in the absence of teeth, 

 both in infancy and old age. 



The use of the incisor teeth is to seize and divide, like scissors, the 

 softer portions of the food. The pointed canine teeth, stronger, and 

 situated at the sides of the dental arches, also cut or pierce the food ; 

 whilst the bicuspids, and especially the molars, or grinders, are em- 

 ployed in bruising, crushing, triturating, and grinding it. The harder 

 parts of our food are broken by the lateral, or posterior teeth only. 

 To accomplish these purposes, the lower jaw is made movable upon 

 the upper one, which has no movement, except in conjunction with the 

 skull itself. By two projections placed at the summit of its back part, 

 named condyles, the lower jaw articulates with the hinder part of two 

 deprevssions in the temporal bones, named the glenoid fossce. The 

 condyles of the lower jaw are flattened before and behind, and widened 

 transversely ; their long diameters are, however, not quite transverse, 

 but are inclined backwards and inwards, so that lines passing through 

 them, would meet at a point further back in the skull. Each condyle 

 has a loose hinge and gliding movement, in the corresponding glenoid 

 fossa ; but the two together form a firm hinge-joint, admitting also of 

 movements, in which both condyles glide a little forwards and back- 

 wards, out of and into the fossae. Moreover, when this motion is 

 limited to one condyle, the lower jaw and teeth move sideways under 

 the upper ones, to the right hand or to the left, the point of the chin 



