496 THE POPULAR SCIENCE MONTHLY. 



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 sneez- 

 ing. 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 press- 

 ure 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 impres- 

 sion 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. 







THE CHEMISTEY OF COOKEEY. 



By W. MATTIEU WILLIAMS. 



XXII. 



I NOW come to a very important constituent of animal food, al- 

 though 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 con- 

 solidated curd of milk. 



It is evident at once that casein must exist in two forms, the solu- 

 ble 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 



