The leverage of the lower human jaw : (an excerpt from the author's note-book) / by John Gorham.

  • Gorham, John.
Date:
1875
Catalogue details

Licence: Public Domain Mark

Credit: The leverage of the lower human jaw : (an excerpt from the author's note-book) / by John Gorham. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

    8/16 (page 6)
    Fig-. 1.—Right half of lower jaw-hone with its complement of eight teeth. F, Condyle—i.e., position of fulcrum, p, Anatomical angle of jaw. F, Point of power. W, Point of weight—a line let fall perpendicularly to a point between two posterior molars. FTP, True lever-rod, regardless of bend of jaw at p. SS', Circular path of lower anterior incisors on a radius FA, showing that they move forwards and upwards until they impinge on upper incisors I. I)D\ Circular path of lower anterior incisors on a radius FA, showing that they move upwards and backwards after impinging on upper incisors I: supposing the condyle to be at B— that is, in a line with the lower teeth, as in the tiger, cat, etc. fulcrum to the weight which strikes the line of direction of the forces at right angles. This ideal line, FW, therefore, represents the true lever of calculation, and is proceeded with according to the ordinary- rule for calculating lever powers. In this manner the bending which takes place at the angle goes for nothing. In order to arrive at legitimate results it is essential that the anatomical points should he very accurately measured on the jaw itself, and it is equally essential that their distances be as carefully measured on the lever-rod; a slight mistake in either of these par- ticulars would lead to error. The shifting of the point of power or the point of weight, even only the one tenth of an inch, would so modify the answer—i.e., the power in grains required to elevate the lower jaw by lever-force—as to render the calculation useless. 1. The rod itself is a line drawn from the fulcrum, F, to the point point of weight, W (Fig. 1). 2. The fulcrum is a line drawn from condyle to condyle (Fig. 2), and is taken at/midway between the two. 3. The point of power, P, is in a line stretched horizontally from the coronoid process to the condyle, and one third of the distance from the former to the latter process. This point is 0 7 of an inch from the fulcrum, FP (Fig. 1), and./p (Fig. 2). 4. The point of weight, W (Fig. 1) is the centre of a line passing through the centre of gravity. 5. The centre of gravity. C (Fig. 2), is found by stretching a thread across from side to side, either in front of the alveoli of the posterior wisdom teeth, or across the centre of the alveoli of the molars. Owing to the variable thickness of this bone, especially