1211 



The grinding faces of the upper bicuspids and molars curve progressively upward 

 and point outward, the first molar being at the lowest point of the curve, the third 

 molar at the highest. The curve of the lower dental arch is the reverse, the first 

 molar at its deepest part, the third molar at its extremity. The greater the depth 

 to which the upper incisors overlap 

 the lower, the more marked this 

 curve and the more pointed are the 

 cusps of the grinding teeth. 



The movement of the human 

 mandible is forward and downward, 

 the resultant of these directions 

 being an oblique line, upon an 

 average of 35 degrees from the 

 horizontal plane. 1 



When the mandible is advanced 

 until the cutting edges of the in- 

 cisors are in contact, the jaws are 

 separated, but at the highest point 

 of the lower arch its third molar 

 advances, and meets and rests upon 

 a high point, the second molar of 

 the upper arch, and thus undue 

 strain upon the incisors is obviated. 



In the lateral movements of the 

 mandible but one side is in effective 

 action at one time; the oblique 

 positions of the cusps of the oppo- 

 site teeth are such that when either 

 side is in action the other is balanced 

 at two or more points. 



Pulp Cavity. 



Root. 



FIG. 923. Vertical section of a molar tooth. 



FIG. 924. Vertical section of a tooth in situ (15 diam- 

 eters), c is placed in the pulp cavity, opposite the cervix, 

 or neck of the tooth; the part above is the crown, that below 

 is the root (fang). 1. Enamel with radial and concentric 

 markings. 2. Dentine with tubules and incremental lines. 

 3. Cementum or crusta petrosa, with bone corpuscles. 4. 

 Pericemental membrane. 5. Bone of mandible. 



There is an anatomical correspondence between the forms and arrangement 

 of the teeth, the form of the condyle of the mandible, and the muscular arrange- 

 ment. Individuals who have teeth with long cusps have the head of the bone much 

 rounded from before backward, and have a preponderance of the direct over the 

 oblique muscles of mastication, and vice versa; teeth with short or no cusps are 

 associated with a flattened condyle and strong oblique muscles. 



Very great aberrations in the dental arrangement are frequently followed by 

 accommodative changes in the condyles of the mandible. 



Structure of the Teeth. A longitudinal section of a tooth will show the presence of a central 

 chamber having the general form of the crown of the tooth, and called the pulp chamber or 

 pulp cavity (cavum dentis). The solid portion of the tooth exhibits three hard tissues: one, the 



W. E. Walker, Dental Cosmos, 1896. 



