THE FORMS, ORIGIN, AND DEVELOPMENT OF THE TEETH. 115 
of human molars. The probability is that the cementum did 
spread over the ancestral forms of the human teeth, but that it has 
been so persistently worn away for so long a time that it has lost 
the tendency to cover this part of the tooth. There is some con- 
formation of this supposition in the teeth of the wappiti deer. 
You see that the cementum does form over the side of the 
tooth, as in the horse ; and I suppose it did extend all over the 
crown just as in this specimen of the tooth of a young horse just 
cut through the gum. But in the deer the cementum is now nearly 
gone, although enough remains to show the tendency to form. 
Although I have said that teeth are not bones, the cementum 
is osseous in character, being formed by membranous ossification 
‘upon the roots of human and some other teeth, and by ossification 
in a fibrous cartilage, according to Magitot, in the teeth of horses, 
&c. The vascular and nerve supply in the cementum is derived 
partly from the pulp, but in the greatest degree from the alveolar 
dental periosteum. The pulp of the tooth is really the slightly 
metamorphosed dental germ now enveloped by the hard structures 
of the tooth. The pulp is made of a mucoid, gelatinous matrix, con- 
taining abundant cells, which are most numerous near the peri- 
phery. The pulp is firmer and denser on its surface than in its 
body, which gives it the appearance of being covered by a definite 
membrane. ‘The vessels of the pulp are very numerous ; indeed, 
it is almost wholly composed of vessels. ‘Three or more arteries 
enter the apical foramen and spread out into branches, which 
finally form a capillary plexus. No lymphatics are known to occur 
in the pulp. There enter by the same apical foramen one large 
and three or four smaller nerves, which, after pursuing a parallel 
course and giving off branches which anastomose but little, 
eventually form into a rich plexus. The nerve supply in the 
dentine is wholly derived through the pulp, and that of the enamel 
as well, if any is found in this substance. 
How the teeth arise and are formed is not yet quite made out. 
About the year 1837, Goodsir told us, in so precise a manner that 
it seemed there could be but little more to learn, exactly how the 
germs of the teeth arose, and how they afterwards developed. 
Goodsir’s opinions got into all our text books, where they remain 
with few exceptions to this day, and are quoted and taught as if 
they had not been contradicted by later and more accurate investi- 
gation and shown to be quite untenable. Instead of the matter 
being so easily settled, the most patient investigation by some of 
the best minds in Europe and America has not yet determined 
many of the most important questions connected with the subject. 
What we know for something like certainty I will state in as concise 
a form as possible. 
In the epithelium there arises what is termed the enamel organ. 
