122 CONTROL OF RESPIRATION. 



produced by which the skilled physician can tell much as to 

 the condition of the respiratory organs. 



The Control of Respiration. Breathing is an involun- 

 tary act. Still we can modify it. We can hold the breath 

 for a time ; but it is stated that one cannot hold the breath 

 long enough to produce death by suffocation. 



The muscles of respiration are under the control of nerves. 

 The center of respiratory control is believed to be in the 

 lower portion of the spinal bulb. This Respiratory Center is 

 one of the most vital points in the body ; for if it is de- 

 stroyed, breathing is completely stopped, and death ensues. 

 This center is affected by the condition of the blood. For 

 instance, if the blood going to this center has not enough 

 oxygen, the center hastens the process of breathing by nerve 

 impulses sent to the muscles of respiration. 



The diaphragm is under the control of the phrenic nerves, 

 which arise from the third, fourth, and fifth cervical nerves. 

 If the neck is broken above the point where these nerves are 

 given off, death almost always immediately follows, because 

 the connection of the respiratory center and the diaphragm is 

 broken. 



COMPOSITION OF THE AIR. 



Dry air contains, by volume, about as follows : 



Oxygen 21.00 



Nitrogen 79.00 



Carbon Dioxid . .04 



100.04 



Various gases are found in the air in small quantities. 

 About one per cent of what is classed as nitrogen in the above 

 table is now known to be a distinct element called Argon. 



If we would understand the results of respiration, how the 

 air does its proper work in our bodies, and how our bodies 



