SNIFFING; SUCKING. 



225 



however, closed, and by these means the whole stream of 

 air is made to enter by the nostrils. The alto 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 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, 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 pnll down the os hyoides and tongue, 

 to the power which draws the handle. 



In the preceding account of respiratory actions, the 

 diaphragm 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 impiratory actions, the 

 other muscles of inspiration (p. 195) are also more or less 

 engaged; and in expiration, the abdominal muscles are 

 assisted by others, previously enumerated (p. 199) as 

 grouped in action with them. 



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, even in sleep. But it is also necessary that 

 respiration should be to some extent under the control of 



Q 



