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margins, it remains to study each tooth much more carefully by cutting sections in the positions of cavity margins and examining the structure as shown in the sections. In this I have not been able to do as much work as I had hoped, but I think there is enough to show that the continuance of the plans may bring knowledge of structure and manner of practice closer together.

The first tooth selected was a superior first bicuspid. With a diamond disk it was cut through from mesial to distal, as shown in Fig. 5, of the occlusal surface. Fig. 6 shows the mesial surface. The pieces were then cut off and ground, so as to leave the sections from the buccal and lingual pieces as close to the cut as possible. Unfortunately the central piece was lost after grinding. In Fig. 6

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a slight discolored spot will be seen at what was the contact point. Fig. 7 shows the buccal section. The enamel was so much affected, and the cementing substance so dissolved out from between the rods at the spot shown, that the tissue went to pieces in grinding, though the surface was unbroken when grinding was commenced, and the action on the dentin can be clearly seen. Notice the position of the oblique grooves and the enamel forming the mesial marginal ridge. If this cavity were prepared as a simple proximal cavity, the occlusal wall would require to be inclined occlusally about 10 centigrades from the horizontal; the gingival wall extended gingivally and inclined about 6 centigrades gingivally from the horizontal. The fallacy of such preparation is well illustrated in Fig. 8, showing the structure of the marginal ridge over to the oblique groove. The cavity wall reaches the area where the rods are becoming very much inclined occlusally, passing over the marginal ridge, and that portion of enamel is practically unsupported and will break out from the

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force coming on it from occlusion, and that is the way it acts in practice.

A second bicuspid was cut in the same way.

The occlusal sur

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face was very perfect. Figs. 9 and 10 show the position of the cuts from the occlusal and the mesial. In these grindings there is little of special interest except the perfection of structure in the oblique

grooves, and the extent to which the apparently slight decay has affected the tissue at the contact points, as shown in Fig. 11, the cementing substance being so softened that the tissue has broken out in grinding. Again, the lingual piece is too much across the rods to show them well. I wish to call attention again, however, to the similarity of structure over marginal ridges and the tips of cusps, as shown in the earlier bucco-lingual sections.

A third bicuspid was cut, attempting to bring the sections in the line of large compound mesio or disto-occlusal cavities. The tooth as sawed is shown in Fig. 12 from the occlusal, and in Fig. 13 from the proximal. The buccal and lingual pieces were ground so as to show the buccal and lingual walls of a disto-occlusal cavity,

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and the mesial and distal pieces were ground so as to show the lingual sides. Notice the appearance of the mesial pit as it will be shown in the section. Of these only the lingual wall of the distoocclusal cavity, and the section from the mesial piece, which shows the mesial pit, are reproduced.

Fig. 14 shows the lingual wall, and may be passed, stopping only to call attention to the outer straight and inner twisted arrangement of the rods on the occlusal as shown in higher power (Fig. 15).

The mesio-lingual piece (Fig. 16) shows the decay in the pit and the structure of the mesial marginal ridge, and Fig. 17, a higher power, shows the rod-directions better. This is a splendid illustration of what may occur when margins approach a marginal ridge. Notice the direction of the rods as the tip of the dentin ridge is approached. In that position it would be impossible not to have unsupported rods which would be sure to loosen sooner or

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