RESPIRATION. 201 



Singing. Singing resembles speaking in the manner of its produc- 

 tion; the luryngetil muscles, by variously altering the position and degree 

 of tension of the vocal cords, producing the different notes. Words used 

 in the act of singing are of course framed, as in speaking, by the tongue, 

 teeth, lips, etc. 



Sniffing. Sniffing is produced by a somewhat quick action of the 

 diaphragm and other inspiratory muscles. The mouth is, however, closed, 

 and by these means the whole stream of air is made to enter by the 

 nostrils. The alge nasi are, commonly, at the same time, instinctively 

 dilated. 



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

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



Laughing. Laughing is a series of short and rapid expirations. 



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

 preceding actions in being 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 character of yawn- 

 ing depends probably on the fact that the muscles concerned are them- 

 selves at all times more or less involuntary, 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 equi- 

 librium, therefore, can be restored only by the entrance of something 

 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. 



Influence of the Nervous System in Respiration. Like all 

 other functions of the body, the discharge of which is necessary to life, 

 respiration must be essentially an involuntary act. Else, life would be in 

 constant danger, and would cease on the loss of consciousness for a few 

 moments, as in sleep. But it is also necessary that respiration should be 

 to some extent under the control of the will. For were it not so, it would 



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