Stregnth of Human Jaw

ATEN-POUND bite requires a forty-pound contraction of the human jaw muscles. That is because the jaws are built on the principle of a pair of tongs, The power is applied near the joint, while the work is performed at the opposite extremity of the jaw levers. Had a mechanical engineer designed the human frame, he would no doubt have built the jaws after the fashion of a nut cracker, with the muscles placed at the ends of the jaw levers, and the teeth between these ends and the joint or fulcrum, so that a powerful bite could be obtained with a very small expenditure of muscular effort. However, in the physiology of man and all other animal life, for that matter, mechanical advantage weighs for little in the presence of other broader considerations. This is why nearly all the principal muscles of the body must be far more powerful than would be necessary were they to act directly upon the work. When chewing we have an almost direct application of the power of the muscles, as they nearly overlie the third molars. There is a vast difference between chewing and biting. In order to determine the average strength of the jaws, Dr. G. E. Black, president of the Chicago Dental University, some time ago devised an instrument of very simple design but with a name that would put the average jaw to a severe test the gnathodynamometer. With this instrument he made gnathody-namometric tests of the jaws of a thousand persons. They were selected at random, and probably represent a good average of civilized people.

 He found that the limit of the bite was not determined by the muscles, but by the teeth themselves. In nearly every case, the patient would cease his efforts, owing to the fact that his teeth hurt. This would depend largely upon the condition of the peridental membranes, and upon the habits of the patient in accustoming his teeth more or less to chewing hard foods. The average of the thousand persons showed 171 pounds for the molar teeth and much less for bicuspids and incisors. Out of the one thousand persons, seventeen developed a force equal to the full registry of the instrument, which was 275 pounds. In a recent publication, Dr.. Black has tabulated the records of about fifty persons who were tested with the gnathodynamometer. The list includes men and women of all classes, from a blacksmith to a Chinese laundryman, and from a servant girl to a. music teacher. It is natural to picture a butcher as a man with a well-developed and powerful jaw, hut among the patients in the list, the butcher showed a power of only 165 pounds with his molars, from which we may infer that he is accustomed to selecting' tender meats for use on his own table. The man with the highest record was a printer who registered 270 pounds; (but any compositor will admit that printers' pie is tough) while the second honors went to a dentist with 240 pounds. Among the women the highest record reached was 160 pounds, while the lowest was 45. Strangely enough, the schoolgirl who made the latter record was able to register 70 pounds with her incisors, which might indicate that she had accustomed her dental membranes of the front teeth to harder work by biting a pencil, or something of the sort. At any rate she was one of those unfortunates who “bite off more than they can chew.” In this list of records, males showed an average of 150 pounds with the molars and 83 with incisors, and females 108 pounds with the molars and 57 with incisors. The instrument with which these records were made comprises a pair of steel arms, which are normally spread apart by spring tension.' Each arm is equipped with a rubber pad, on which the biting is done. When the patient bites the pads, the levers are forced toward each other and cause an index needle to travel over a graduated arc which shows the force of the bite in pounds. One of the difficulties that had to be contended with was that the full power of the muscles cannot be exerted until the jaws are nearly closed. This made it necessary to reduce the biting pads to a minimum compatible with safety. In selecting patients whose jaw muscles were to be tested, it was imperative that the teeth be examined for any indications of decay which would weaken them to such an extent as to cause injury when biting on the in- strument. In addition to the gnathodynamometer, Dr. Black has invented another instrument with an equally terrifying name — the phagodynamometer, with which he tests the resistance to chewing offered by various foods. This instrument comprises a pair of plungers with toothed surfaces at their adjacent ends. By means of a lever one of the plungers is forced against the other. The latter acts upon a spring, connected with an index needle arranged to sweep over a dial. The pressure of one plunger upon the other plunger may then be read on this dial. With this instrument, some important studies have been made of the force required in the mastication of food. These are shown graphically at the top of this page. In place of muscles the jaw is represented as being provided with a scale beam and a 10-pound sliding weight on the beam. If the weight be moved to the point marked 50, a fifty-pound crushing force will be exerted at the second molars. The tenderest meat was found to be boiled tongue, the central part of which offered a re- Rear vjew 0f the device. sistance of from three to five pounds. Boiled tongue, when cold, offered a resistance of from fifteen to twenty pounds. In order to crush pork chops, loin, a force of from twenty to twenty-five pounds was necessary. Mutton chops required from thirty to forty pounds; beef, round, from forty to fifty, and beefsteak, well done but rather tough, from sixty to eighty pounds. An experienced butcher selected some very tough meats, from the neck of an old animal, which stood up against a force of from seventy to ninety pounds before the crush occurred. These tests were made a number of years ago, before storage meat was used to the extent it is now. The figures should be reduced about one-third for meat made tender by storage. With these data, the mathematically inclined may proceed to figure out the horse-power developed at the boarding-house table. A surprising fact brought out by the use of the phagodynamometer is that the hardness of food does not determine its danger to the teeth. For instance, candies that are quite hard offer much less danger to the teeth than gum drops, which ordinarily are mashed out of shape at from twenty to thirty pounds. If part of the gum drop became wedged between the cusps of the teeth, it was found that frequently it could not be completely crushed with a pressure of less than 250 pounds. Sticks of licorice proved particularly dangerous in this way. But the most remarkable discovery was that even bread might cause the breaking of a cusp. To quote Dr. Black, “With my personal observation, more teeth that seem sufficiently strong have been broken with bread crusts, and not very hard crusts either, than with any other one thing. I used sometimes to feel that persons relating these accidents were not quite honest in their statements, but when I tried the bread and found that it would wedge in between the cusps of the teeth, and not be crushed out with a force of 350 pounds, I changed my mind,"