It4 THE MICROSCOPICAL NEWS. 
branches at a little distance beneath the surface ; consequently, in 
longitudinal sections the tubes are seen running from the pulp 
cavity toward the periphery. But in a transverse section across the 
neck of the tooth there may be seen the ends of the tubes where 
they present the appearance of the ends of pipe stems fused 
together. ‘The number of tubes in each tooth is so great that no 
one has to my knowledge undertaken to estimate them. Near the 
pulp they are closely packed together, but near the outside of the 
tooth they are more widely separated. ‘The dentinal tubes do not 
pursue a straight line, but describe curves, some of the larger of 
which have been compared to the italic letter # There can be no 
doubt that in many cases the course of the tubes is that of an 
elongated spiral; and I fancy this is really the course they always 
take. Not only do the tubes divide as they pass outwards, but 
their branches anastomose with the branches of other tubes, thereby 
forming loops. In the crown of the human tooth there are but 
few branches except near the enamel, but in the root they are more 
numerous. Sometimes the tubes pass beyond the dentine, and 
anastomose with the canaliculi of the cementum. In marsupials 
and some other animals the enamel is constantly penetrated by the 
tubes, and not unfrequently they are seen in the human enamel. 
Mr. Tomes is surprised to find the human enamel penetrated by 
the tubes, but this I take to be a “harking back” to a former 
type. There is what appears to be an excellent illustration of this 
reversion in the last number of the ‘‘Cosmos,” an American 
dental journal. Dr. Knowlton, of Belden, Ohio, records a case 
of abnormal dentition, where a tooth with five cusps came in 
the place of a central incisor. If you will examine the bicuspid 
teeth in a human jaw you will see they are made up of a combination 
of cones, three in front and three behind. Then look at the canine 
teeth, and you will see they are modifications of the bicuspids by 
a depression of the inside cusps. ‘Then examine the incisors, and 
you will see the same outlines of the three cones as on the outside 
of the bicuspids, showing they are modifications of teeth that were 
at one time not wedge-shaped but multi-cuspid. Now, unite the 
middle cone of this abnormal tooth with the two cones on each 
side of it, and depress the other two and you have an incisor. Go 
back still further, and you find among fishes the aplodactylus 
has teeth made up of three-cusp combinations, each of which is 
very similar to this abnormal tooth except in the number of its 
cusps. In the primates, carnivora, &c., the cementum is confined 
to the roots of the teeth: but it runs through the bodies of the 
teeth of the herbivora, as I have shown. It is thought the 
cementum has a zendency to form also over the crowns of the 
human teeth as well as in the animals named, and I think this 
hypothesis is correct, for it is sometimes found in the depressions 
