452 
REVIEWS. 
the substance of each tooth. It is principally different from bone in con* 
taining more phosphate of lime ; or according to some, fluoric acid in com- 
bination with lime. The cement is softer than either of the other compo- 
nents, contains more animal matter, and resembles more nearly the common 
bone of the body. Sir Everard Home attributes its produce to an ossifica- 
tion of the secreting membrane of the enamel ; Cuvier considers it as a 
specific secretion. It may perhaps be better explained after this manner : 
the pulp from whence the ivory is secreted is surrounded by a membranaceous 
capsule, and presents two layers ; the central layer secretes the enamel, 
while the outer furnishes the cement, and surrounds the terticeous striae of 
enamel, and unites the other substances into one firm mass.” 
What follows is the Editor’s account of 
THE TEETH. 
“ The substances which enter into the formation of tooth are bone, enamel, 
and crusta petrosa. These three are perfect when the tooth enters the 
mouth, and lay one under the other. The crusta petrosa is the outer cover- 
ing, of a dark colour and tolerable thickness. It was formerly mistaken for 
tartar, covering and dirtying t he teeth ; but that it is not tartar is shown by 
its always covering young teeth, and being absent from the front of old 
horses when the attrition consequent upon gathering the food has worn it off ; 
therefore whiteuess in teeth is no sign of youth in the horse. The crusta 
petrosa is dark, tough, and highly organised substance, covering the whole 
of the fang, and following the enamel wherever it bends or dips into the wind- 
ings upon the table of the tooth. 
“ The enamel lies next under the crusta petrosa, and consists of a thin layer 
of very brittle unorganised, and crystalline substance; it principally 
coats the crown of the tooth, being very thin up the fang, which it only 
partially envelops. 
“ The main bulk of the tooth consists of bone, which is tough, and of a 
yellowish colour, not so dark as the crusta petrosa, but much darker than 
the enamel, which last is perfectly white. It has an opening at the point, 
or bottom of the fang, which leads to a cavity that contains the vessels and 
nerves that nurture the substance of the tooth, as well as endow it with 
feeling. 
“Now, when we properly consider the services to which the horse’s teeth 
are put, viz., to nipping off and grinding up the food, the use of these three 
substances is perceived. The enamel, w r hich is as brittle as glass, yet being 
firmly enclosed between two tough substances, is preserved from chipping 
or breaking ; but being protected acts with the efficiency of steel. It is 
usually the least worn of the three materials that enter into the composition 
of the tooth, projecting comparatively far beyond them.” 
Let us take an example of amplification, from the de- 
scriptions of the bones : 
“ The navicular , nut , or shuttle bone , is situated at the posterior part of the 
coffin, to which it is articulated, as just noticed : its upper surface forms a 
continuation of the articulating surface of the coffin, by which these bones 
jointly receive the broad extremity of the little pastern. The navicular bone 
is, as it were, laid upon the flexor tendon, which passes up over its posterior 
edge : all which parts will be more particularly considered when we describe 
the feet.” 
Thus far Blaine ; next comes his editor’s account : — 
“ The nut , the shuttle , the quiltor , or the vanicular bone , is situated behind 
