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Page:Popular Science Monthly Volume 24.djvu/512

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ing most closely resembles sneezing, not only as regards its origin, but also as regards its execution. This is a reflex action which follows an irritation of the air-passages, particularly of the windpipe and the larynx, but also of the pharynx and the nasal cavity. Stimulation of other nerves, as those of the skin by a draught of cold air, may also produce coughing. The expiratory impulses induced may attain great violence, so as in this respect to resemble the single impulse of sneezing. While, however, in sneezing, the stream of air escapes, as a rule, through the nose, in coughing it escapes through the cavity of the mouth, which is shut off by the raised soft palate from the nasal cavity, and enlarged by dropping the lower jaw, and by the depression of the floor of the cavity, the tongue at the same time being pushed forward. The closed glottis holds this air back for an instant against the pressure of the abdominal muscles, and then suddenly opens part way, letting it escape with an explosive noise, generally accompanied by a sound, shrill or deep as the case may be, produced by the vocal chords. Performed voluntarily, and with less violence, coughing assumes the form known to us as "clearing the throat." In laughing, the separate expiratory impulses are not so violent, and the stream of air passes through the fairly open mouth, or, when the mouth is shut, through the nose. It is accompanied by contractions of the muscles of the face, and is mainly involuntary, being generally caused by an impression produced upon the higher parts of the brain. Violent laughing may be caused by tickling some parts of the body. Characteristic sounds are produced in the same way as already described in coughing, and in both, when long continued, the air which from time to time is quickly inspired may produce a clear, shrill note in passing through the glottis.



I NOW come to a very important constituent of animal food, although it is not contained in beef, mutton, pork, poultry, game, fish, or any other organized animal substance. It is not even proved satisfactorily to exist in the blood, although it is somehow obtained from the blood by special glands at certain periods. I refer to casein, the substantial basis of cheese, which, as everybody knows, is the consolidated curd of milk.

It is evident at once that casein must exist in two forms, the soluble and insoluble, so far as the common solvent, water, is concerned. It exists in the soluble form, and completely dissolved in milk, and insoluble in cheese. When precipitated in its insoluble or coagulated