200 PRINCIPLES OF PHYSIOLOGY 



necessity for calling into play the expiratory 

 centre except occasionally when there is 

 more or less obstruction to the free exit of 

 air from the lungs. This explains the 

 mechanism of coughing, which consists of 

 violent expiratory efforts. But the inspira- 

 tory centres may be influenced through other 

 nervous channels. Strong stimulation of 

 almost any sensory nerves will cause inspira- 

 tions. Slapping the skin with a wet cloth, 

 plunging into cold water, a sudden draught of 

 cold air, pain, will usually cause inspirations. 

 Probably the first breath of a newly-born 

 child is thus excited. Finally, impulses may 

 come to the respiratory centres from the 

 higher centres in the brain. Thus, within 

 narrow limits, we can voluntarily control the 

 breath. When by certain morbid changes in 

 the higher centres, there is unconsciousness, 

 breathing may still go on, but in a curious, 

 fitful way, as if a mechanism regulating the 

 respiratory centres had been interfered with. 

 So that the respiratory centres are maintained 

 in a condition of physiological equilibrium by 

 numerous nervous impulses coming to them by 



