RESPIRATION. 281 



action of the diaphragm and other inspiratory muscles. The mouth is 

 closed, and the whole stream of air is made to enter the air-passages 

 through the nostrils. The alae nasi are commonly at the same time 

 instinctively dilated. 



Sobbing. Sobbing consists of a series of convulsive inspirations, at 

 the moment of which the glottis is usually more or less closed. 



Laughing. Laughing is made up of a series of short and rapid expi- 

 rations. 



Yawning. Yawning is an act of inspiration but is unlike most of 

 the preceding actions as it is always more or less involuntary. It is 

 attended by a stretching of various muscles about the palate and lower 

 jaw, which is probably analogous to the stretching of the muscles of the 

 limbs in which a weary man finds relief, as a voluntary act, when they 

 have been some time out of action. The involuntary and reflex charac- 

 ter of yawning probably depends on the fact that the muscles concerned 

 are themselves at all times more or less used involuntarily, and require, 

 therefore, something beyond the exercise of the will to set them in 

 action. For the same reason, yawning, like sneezing, cannot be well 

 performed voluntarily. 



Sucking. Sucking is not properly a respiratory act, but it may be 

 most conveniently considered in this place. It is caused chiefly by the 

 depressor muscles of the os hyoides. These, by drawing downward and 

 backward the tongue and floor of the mouth, produce a partial vacuum 

 in the latter : and the weight of the atmosphere then acting on all sides 

 tends to produce equilibrium on the inside and outside of the mouth as 

 best it may. The communication between the mouth and pharynx is 

 completely shut off by the contraction of the pillars of the soft palate 

 and descent of the latter so as to touch the back of the tongue; and the 

 equilibrium, therefore, can be restored only by the entrance of some- 

 thing through the mouth. The action, indeed, of the tongue and floor 

 of the mouth in sucking may be compared to that of the piston in a 

 syringe, and the muscles which pull down the os hyoides and tongue, to 

 the power which draws the handle. 



The Nervous Apparatus of Respiration. 



Like all other functions of the body, the discharge of which is nec- 

 essary to life, respiration is essentially an involuntary act. Unless these 

 were the case, life would be in constant danger, and would cease on the 

 loss of consciousness for a few moments, as in sleep. It is, however, 

 also necessary that respiration should be to some extent under the con- 

 trol of the will. For were it not so, it would be impossible to perform 

 those respiratory acts which have been just discussed, such as speaking, 

 singing, and the like. 



