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106 DENTAL IHSTOLOGY AND OPERATIVE DENTISTRY.

arc ()verlaj)jH'd lor a short distance by tlic ccinciituiu. Tliis inclination
grows less and less, and at some ])lacc in tlie gingival liail'oftlic middle
third of the surface they arc in the horizontal plane. At this point they
are also usually per])endi('ular to the surface of the enamel. J^issing
from this point they bcconu; inclined more and more occliisally from the
horizontal |)lanc, at the junction of the occ^liisal and middle thirds about 8
to 12 centigrades (28° to 40°) in bicuspids and molars, and 8 to 18
centigradcs (28° to 65°) in incisors and canines. In the occlusal third
the inclination increases rapidly, and often the outer ends of the rods

Fig. 88.

Diagram of enamel rod directions, from a photograyih of a buccolingual section of a superior
bicuspid.
are inclined more than the inner ends. Over the point of the cus])s
and the crest of the marginal ridges the rods reach the axial plane,
though they are often very much twisted about each other in the inner
half of their length. This position does not always correspond with the
highest point of the cusp, but is inclined slightly axially from that j)osi-
tion, and corresponds with the highest ])()int of the dentin cusp.
Passing down the central slope of the cusp, or ridge, the rods
become again inclined aAvay from the axial plane toward the groove, or
pit, leaning toward each other where the two plates meet. The degree of
inclination of the rods on the central slope of the cusps de])ends upon the