316 THE WORSE. 
THE LOWER JAW. 
THE LOWER Jaw resembles in general shape the letter V, the point 
being rounded off forwards and receiving the incisor teeth, while the 
two branches are broad, thin, and slightly curved, being surmounted by 
the condyles for articulation with the glenoid cavity of the temporal bone, 
and giving lodgment to the tushes and grinders. The two lines of 
alveolar cavities are not so wide as those of the upper jaw, and conse- 
yuently the two sets of teeth do not exactly correspond. 
THE TEETH. 
THE TEETH are developed within their appropriate cavities or sockets, 
which are found exactly corresponding with their number in the apper 
and lower jaws, being narrower in the lower than in the upper. Before 
birth they are nearly all in a state of incomplete growth, covered and con- 
cealed by the gums, but soon afterwards they rise through it in pairs, the 
first set, or milk teeth, being in course of time superseded by the perma- 
nent teeth as in all the mammalia. The following is the formula of the 
complete dentition of the horse :— 
Incisors 8, canine 2, molars 32. 
EacH TootH is developed within its corresponding cavity in the jaw, 
and is made up of three distinct substances—cement, enamel, and dentine. 
The cement of the horse’s tooth (sometimes called crusta petrosa) closely 
corresponds in texture with his bone, and, like it, is traversed by vascular 
canals. The enamel is the hardest constituent of the tooth, and consists 
of earthy matter arranged in the animal matrix, but contained in canals, 
so as to give the striated appearance which it presents on splitting it 
open. Dentine has an organized animal basis, presenting extremely 
minute tubes and cells, and containing earthy particles, which are partly 
blended with the animal matter in its interspaces, and partly contained 
in a granular state within its cells. ‘These three substances are shown in 
the annexed sections of an incisor tooth, one of which, Fig. 4, is of the 
natural size, while the other, Fig. 5, is shown under the microscope— 
IN THE MoLAR teeth the arrangement of these three substances is the 
same, except that the cement and enamel dip down into two or more 
cavities instead of one, and are also reflected in a sinuous manner upon 
the sides. This inequality in the hardness of the component parts of 
these teeth causes them to wear away with different degrees of rapidity, 
and thus leaves a rough surface, which materially aids in grinding down 
the hard grain which forms a large portion of the animal’s food. In the 
upper jaw, the table presented by each molar tooth is much larger than 
those of the corresponding lower teeth, and therefore it is easy to dis- 
tinguish the one from the other. 
THE TEMPORARY OR MILK INCISORS differ in shape a good deal from 
the permanent set. The milk teeth are altogether much smaller, but 
especially in the neck, which is constricted in them, whilst in the perma- 
nent set, which go on growing as they wear out, the diameter is nearly the 
same throughout. The former are also whiter in colour, and have grooves 
or indentations on their outer surfaces, running towards the gum. Lastly, 
the mark on the table is much slighter than in the permanent teeth. The 
temporary molars are not distinguishable from the permanent teetb. of 
that class. 
