236 RESPIRATION. 



it may. The communication between the mouth and 

 pharynx is 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, so that the air cannot find 

 entrance by this way ; and the equilibrium, therefore, can 

 be restored only by 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, 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 inspiratory actions, the 

 other muscles of inspiration (p. 206) are also more or less 

 engaged; and in expiration, the abdominal muscles are 

 assisted by others, previously enumerated (p. 209) 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 

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

 perform those voluntary respiratory acts which have been 

 just enumerated and explained, as speaking, singing, 

 straining, and the like. 



The respiratory movements and their regular rhythm, 

 so far as they are involuntary and independent of con- 

 sciousness (as on all ordinary occasions they are), seem to 

 be under the absolute governance of the medulla oblon- 

 gata, which, as a nervous centre, receives the impression 

 of the " necessity of breathing," and reflects it to the 



