The Upper Jaw ([Fig. 3.]) is, besides the Teeth, composed on each side of three Bones. The First (a), into which the Poisonous Fangs are fixt, is articulated with the Anterior Protuberance of the Orbit of the Eye; and has a Motion of Flexion and Extension, that is, forwards and backwards, by which the Fangs are Erected or Depress’d. It is small at the Joint, but grows broader by degrees, to a pretty large Basis, the better to contain a considerable Number of Teeth. It is Spongy like the Substance of the Vertebræ, and no ways fit to be the immediate Organ of Hearing, as Mr. Charas and some others have imagined.
The Second (c), is a broad thin Bone, Articulated by one Extreme to the Former, (f), and by the other firmly fixt to the middle of the third Bone. When this is thrust forwards, it likewise pushes the First, and by this means the Erection of the Fangs is helped; and when it is pull’d backwards, they are depressed.
The third Bone (e d), is join’d by one Extremity (e), to the End of one of the Bones of the Lower Jaw (c, [Fig. 4.]), And being somewhat crooked, turns in a little towards the Basis of the Cranium, and running along the Inferior Part of it towards the Nose, terminates near the Internal and Anterior Part of the first Bone.
The Lower Jaw (c d e f g, [Fig. 4.]) on each side is made up of two Bones, but firmly united, the Extremity of the one entring within the other (f). The First (c d e) articulates with the Second of the Common Bones (b), where it is broad, and sends off an Apophysis, into which there is a Muscle inserted, which helps to open the Jaw. There is in this is a Hole (d), for the Entrance of the Branch of the Nerve, which passing thro’ a Canal in the middle of it, goes to the Extremity of the Second Bone, and in its way sends off several Branches which go to the Teeth; and also a very considerable one, which goes out at (e), and is wholly spent upon the Neighbouring Muscles.
The Second Bone (f g) serves chiefly to receive the small Teeth, which answer to those in the upper Jaw.
As for the Teeth, they are of two Sorts, the Great, or poisonous Fangs, and the Small.
The Great (b, [Fig. 3.]), being fixt in the First Bone of the Upper Jaw, are Crooked and Bent, like the Dentes Canini in most Carnivorous Animals. They are manifestly hollow from their Root a considerable way, not to the very Apex or Point, (which is solid and sharp, the better to pierce the Skin) but to a small distance from it, as is plainly seen by splitting the Tooth thro’ the middle (Vid. [Fig. 6.]). This Cavity ends at the Convex Part in a visible Slit, very well resembling the Nip or Cut of a Pen ([Fig. 9. d]), which is the Emissary or Outlet to the Poyson.
Galen [(37)] has given us a considerable Hint of this Make of the Tooth: For, The Mountebanks (He says) used to suffer themselves to be bit by Vipers, having first with some Pastes stopt the Holes of their Teeth, that the Venom being thus kept in, the Spectators might think they did by their Antidote secure themselves from its dangerous Effects.
The Reason why these Teeth are Crooked, is, That the Point of the Tooth, when the Viper bites, may be Perpendicular to the Part to be Wounded; for the Head being raised back in the Time of Biting, and the Tooth erected, if this were strait, It would not, by reason of its oblique Situation to the part, enter with so much Force, nor so deep into the Flesh.
As for the Number of the Poisonous Fangs, I have observed, that there are, for the most part, besides One, Two or Three on each side, fixt Perpendicularly to the first Bone of the Upper Jaw, some others which are Young, and of a smaller Size, adhering to the same Bone: Their Points are hardened, and they have their Fissures formed as in the other, but their Roots are Soft and Mucilaginous, like the Roots of the Teeth in Infants, and so they lye always depress’d at the Bottoms of the Former, as may be seen [Fig. 10. c].