In the carnivora, and all such as have canines longer than the other teeth, there is at least one vacancy in each jaw, for lodging the cuspidatus of the opposite jaw. There is a vacancy be hind each canine in the bear.
The horned ruminating animals not only want entirely the upper incisors, but they are also destitute of cuspidati, ex cept the stag, which has rudiments of these teeth ; and the musk (moschus moschi fer) where they are very long, and cur ved in the upper jaw.
Between the incisors and grinders of the horse, a very large vacancy is left, in the middle of which a small canine tooth, termed the tusk, is found in the male animal; but very rarely in the fe male.
The elephant has grinders and two tusks in the upper jaw ; but the former only in the lower. The immense tusks belong properly to the male animal, as they are so small in the female, generally speaking, as not to pass the margin of the lip. (Corse, in Phil. Trans. 1799, part 2. p. 208.) The sloths have grinding and canine teeth, without incisors. The dolphin and porpoise have small conical teeth, all of one size and shape, arranged in a conti nued line throughout the alveolar margin of both jaws. The cachalot (physeter macrocephalus) has these in the lower jaw only. The teeth of the seal are all of one form, viz. that of the canine kind ; conical and pointed.
The narwhal has no other teeth than the two long tusks implanted in its os in termaxillare ; of which one is so frequent ly wanting.
The structure of the incisor teeth, in the rodentia, deserves attention on se veral accounts. They are covered by enamel only on their anterior or convex surface, and the same circumstance holds good with respect to the tusks of the hip popotamus. Hence, as the bone wears down much faster than this harder co vering, the end of the tooth always con stitutes a sharp cutting edge, which ren ders it very deserving of the name of an incisor tooth.
This partial covering of enamel refutes, as Blake has observed (" Essays on the Structure, &c. ot' the Teeth," p. 212,) the opinion, that the enamel is formed by the process of crystallization.
The incisor teeth of these animals are used in cutting and gnawing the'harder vegetable substances, for which their above-mentioned sharp edge renders them particularly well adapted. Hence Cuvier has arranged these animals in a particular order, by the name of rodentia, or the gnawers. As this employment subjects the teeth to immense friction and mechanical attrition, they wear away very rapidly, and would soon be con sumed, if they did not possess a power of growth, by which the loss is recom pensed.
These teeth, which are very deeply imbedded in the jaw, are hollow inter nally, just like a human tooth which is not yet completely formed. Their cavity is filled with a vascular pulp, similar to that on which the bone of a tooth is form ed ; this makes a constant addition of new substance on the interior of the tooth, which advances to supply the part worn down. The covering of enamel extends over that part of the tooth which is con tained in the jaw, as we might naturally expect : for this must be protruded at some future period, to supply the loss of the anterior portion. Although these teeth are very deeply implanted in the maxillary bones, they can hardly be said to possess a fang or root ; for the form of the part is the same throughout ; the covering of enamel is likewise continu ed ; and that part, which at one period is contained in the jaw, and would form the fang, is afterwards protruded,to constitute the body of the tooth.
The constant growth of these teeth therefore proceeds in the same manner, and is effected on the same principles, as the original formation of any tooth ; and can by no means furnish an argument for the existence of vessels in the substance of the part.
We cannot help being struck with the great size of these teeth, compared with the others of the same animal, or even with the bulk of the animal. Their length in the lower jaw nearly equals that of the jaw itself, although a small proportion only of this length appears through the gum. They represent the segment of a circle, and are contained in a canal of the bone, which descends un der the sockets of the grinders, and then mounts up, in some instances, to the root of the coronoid process: hence, although their anterior cutting in the front of the mouth, the posterior extremity is behind all the grinding teeth. No ani mal exhibits this structure better than the rat. The beaver also affords a good specimen of it on a larger scale. It has been drawn in this animal by Blake, (" Essay on the structure, &c. of the teeth.") The tooth.does not extend so far in the upper jaw ; it is there implanted in the intermaxillary bone, and terminates over the first grinder.