184 RESPIRATION. 



so than it commonly is in the expulsion of faeces or urine; but 

 in these latter instances also, in cases of great pain and diffi- 

 culty, it may cease to be a voluntary act, and be quite beyond 

 the control of the will. 



In speaking, there is a voluntary expulsion of air through 

 the glottis by means of the abdominal muscles ; and the vocal 

 cords are put, by the muscles of the larynx, in a proper posi- 

 tion and state of tension for vibrating as the air passes over 

 them, and thus producing sound. The sound is moulded into 

 words by the tongue, teeth, lips, &c. the vocal cords produc- 

 ing the sound only, and having nothing to do with articulation. 



Singing resembles speaking in the manner of its production ; 

 the laryngeal 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, &c. 



Sniffing is produced by a somewhat quick action of the dia- 

 phragm and other inspiratory muscles. The mouth is, how- 

 ever, closed, and by these means the whole stream of air is 

 made to enter by the nostrils. The alse nasi are, commonly, 

 at the same time, instinctively dilated. 



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 downwards and backwards the tongue and floor of 

 the mouth, produce a partial vacuum in the latter ; and the 

 weight of the atmo&phere then acting on all sides tends to pro- 

 duce equilibrium on the inside and outside of the mouth as 

 best it may. The communication between the mouth and 

 pharynx is completely shut off, probably 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 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. 



In the preceding account of respiratory actions, the dia- 

 phragm and abdominal muscles have been, as the chief muscles 

 engaged, and for the sake of clearness, almost alone referred 

 to. But, of course, in all inspiratory actions, the other muscles 

 of inspiration (p. 162) are also more or less engaged ; and in 

 expiration, the abdominal muscles are assisted by others, pre- 

 viously enumerated (p. 165) as grouped in action with them. 



