296 
On the Teeth of the Ox^ Sheep, and Pig, 
iilli 
I proceed to speak of the Crusta, the third constituent of a 
tooth. This substance approaches nearer to ordinary bone in 
the arrangement of its component 
i'''S-8-* parts than does either tlie Dentine or 
Knamel. It also resembles bone 
both in its chemical composition 
and in its density, and hence it is 
frequently designated the bone of 
the tooth. The proportionate quan- 
tity of Crusta to the other consti- 
tuents depends upon the kind rather 
than upon the size of the tooth. For 
example, in a simple tooth very little 
is present, but on the contrary in a 
compound one, a good deal of crusta 
exists. Tiiis difference arises from 
the circumstante that the Crusta, in 
a simple tooth, is met with chiefly 
on its fang, while in the other variety 
it not only covers the fang, but dips 
deeply with the Tmamel into the interior of the organ. 
Existing on the fangs of the tooth, and being covered here by 
a membrane which also lines the socket in which it is placed, 
the Crusta helps to retain the tooth in its situation. When ex- 
amined microscopically the arrangement of the component parts 
of tlie Crusta is observed to differ considerably from that of the 
Dentine or Enamel. It is more or less thickly filled with bone 
cells (Jacunre) from which radiate minute canals, called therefore 
the caiialic7iU. (See fig. 11.) Besides these canaliculi the Crusta 
has, in our domesticated animals particularly, numerous passages 
opening on its external border in contact with the lining mem- 
brane of the socket, like the opening of the dentinal tubes into 
the pulp cavity. This arrangement is well seen in sections even 
of young teeth, particularly those of the horse and pig, and dis- 
tinctly proves that a tooth, even at that time, derives some of its 
nourishment from the surrounding parts of its socket. In young 
teeth these external tubes are situated principally about the 
upper part of the fang, as it is here that the Crusta, from the 
greater thickness of tlie Dentine, is furthest removed from the 
pulp cavity, and consequently ffom tlie more common source of 
nourishment. These things will have to be referred to again. 
In addition to the lacuna", their canaliculi, and the external 
order of tubes, the Crusta has also numerous passages v;ithin it of 
* Fig. 8. Section of dentine and enamel, fiom an incisor of a sheep, magnified 
100 diameters, showing the tubes ending iu lacunsc. u, dentinal tubes; D*, lacuna;; 
E, the enamel. 
