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122 DENTAL HISTOLOGY AND OPERATIVE DENTISTRY.
dentin niati'ix. Tliis is lairly iinif'onn thn)U<;li()nt tlu; dentin. Tlie
character of the tubuk's is diffcirent in tlie crown and root portions.
In the crown tlie tnbides hrancli bnt little lhron<>h most of their
course ; but in the outer j)art, close to the enamel, they branch and
anastomose with each other (juitc freely. I i^. 102 shows a held of
dentin just beneath the enamel, as seen with a high jM)wer, and shows
the diameter of the tubides, their branching, and the amount of nuitrix
between one tubule and the next. The relation of one tubule to each
other is shown also in sections cut at right angles to their direction
(Fig. 103). In the crown portion the tubules pass from the pulp chamber

Fig. 104.

Criiwn 111 a ninl.-ir, iiirsii)ili;-lal Sfctimi, shuwiiiL,' iiein.'tratKni ^f caru ;- : .1, carits )iLiiL-truUii,L; ilen-
tin; />, line cif abrasion ; 7', iiiilji cIimiuIjlt. ;Ahoiit jn .)

to the dento-enamel junction in sweeping curves, so as to enter the pulp
chamber at right angles to the surface, and end next to the enamel at

right angles to that surface. This produces S- or F-shaped \ or /]
curves, which are known as the primary curves of the tubules. Through-
out their course the tubules are not straight, but shoAV a great many
wavy curves, known as the secondary curves. These appear as waves
when seen in longitudinal sections, but are really the effect of an open
spiral direction, as is seen by changing the focus of the microscojie in
studying sections cut at right angles to the direction of the tubules.