Volume 1

Human physiology / by Robley Dunglison.

  • Dunglison, Robley, 1798-1869.
Date:
1856
Catalogue details

Licence: Public Domain Mark

Credit: Human physiology / by Robley Dunglison. Source: Wellcome Collection.

Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.

    129/738 (page 129)
    As soon as the food is introduced into the mouth, the jaws are closed to retain it, and subject it to mastication. Frequently, however, they assist in the act of prehension, as when we bite a fruit, to separate a portion from it; the incisor teeth acting, in such case, like scissors. This is chiefly produced by the contraction of the muscles that raise the lower jaw ; and it is probable, that the action of the stylo-hyoideus Fig. 52. Action of the Lower Jaw in Prehension. A. Frontal bone. B. Temporal. C. Parietal. D. Occipital. E. Coronoid process of the lower jaw, to which the temporal muscle is attached. F. Condyloid process or head of the lower jaw. G. Lower jaw. H. Mastoid process. I. Upper jaw. J. Cheeli bone. K. Orbit. L. Meatus auditorius exteruus. L*. Coronal suture. M. Squamous suture. N. Lambdoidal suture, g. Lower jaw depressed. is concerned in the movement; drawing the head and upper jaw with it downwards and forwards. The levator muscles of the jaw act here with great disadvantage;—the lower jaw representing a lever of the third kind; the fulcrum being in the joint; the jiower at the insertion of the levator muscles; and the resistance in the substance between the teeth. The arm of the resistance is, consequently, the whole length of the lever; and we can understand why we are capable of developing so much more force, when the resistance is placed between the molares; and why old people,—who have become toothless, and are, consequently, constrained to bite with the anterior part of the jaws,—the only por- tion that admits of contact,—cannot bite with any degree of strength. The size of the body, put between the incisor teeth, influences the degree of force that can be brought to bear upon it. When small the force can be much greater, as the levator muscles are inserted perpen- VOL. I.—9