180 THE BODY AT WORK 



victim of the blow feels as if he would never again be able to 

 draw breath. 



Modified Respiratory Movements. The object of coughing 

 is to expel foreign matter from the windpipe or larynx ; of 

 sneezing, to clear the nose. The former action consists of a 

 long deep inspiration ; the closure of the glottis ; a forcible 

 expiration. The blast of air encountering a closed glottis 

 acquires considerable pressure. When the resistance of the 

 glottis is overcome, the blast rushes through, carrying with it 

 mucus or bread-crumb, or whatever the substance may be 

 which irritated the endings of the superior laryngeal nerve. 

 In sneezing, the back of the tongue is thrust against the palate, 

 closing the aperture of the fauces. Inspiration is prolonged. 

 A strong expiration follows. The blast rushes through the 

 nasal cavities. This reflex is usually provoked by a tickling 

 of the endings of the fifth nerve in the nasal mucous mem- 

 brane. It is also caused in many persons, through the optic 

 nerve, by a bright light ; an apparently purposeless reflex about 

 which we shall have something more to say in a subsequent 

 chapter. Laughing and crying are modified respiratory move- 

 ments as useless, so far as any immediate purpose is accom- 

 plished, as sneezing in response to a bright light. As means 

 of expressing emotions they have been cultivated by the 

 human race. Possibly a case for crying might be made out 

 on physiological grounds. Under certain circumstances it 

 relieves a feeling of distress which, while it lasts, is detrimental 

 to the proper functions of the body. Laughing undoubtedly 

 is beneficial. The rapid movements of the chest quicken the 

 circulation. The shaking of the midriff favours the discharge 

 of digestive secretions, accelerates the movements of the alimen- 

 tary canal, and generally is beneficial to digestion. But 

 " laugh and grow fat " is not necessarily the order of cause and 

 effect. An efficient digestion and a good capacity for assimila- 

 tion lead to a sense of bien-etre which predisposes to a merry 

 view of life. 



Yawning is a deep inspiration with open mouth and larynx. 

 It commences usually at the end of a normal inspiration, a 

 slight pause being followed by further inspiration, deep and 

 prolonged. Its commencement seems to be due to impulses 

 generated by the relaxation of the tone of the muscle which 



