140 ANATOMY FOE NURSES. [CHAP. XII. 



connected together, any change in the blood immediately affect- 

 ing the respiration. 



It would appear that stimulation of the respiratory centre in 

 the medulla oblongata depends primarily upon the condition of 

 the blood. If the blood is very rich in oxygen the respirations 

 are feeble and shallow ; if, on the other hand, the blood is highly 

 venous the respirations are deeper and more frequent, and if the 

 blood remains venous, gradually become forced and laboured 

 until we get the condition called " dyspnoea." Should the blood 

 get more and more venous, the impulses generated in the respir- 

 atory centre become more and more vehement. These nervous 

 impulses, instead of confining themselves to the usual nerves 

 distributed to the ordinary respiratory muscles, overflow on to 

 other nerves and put into action other muscles until there is 

 scarcely a muscle in the body that is not affected. The muscles 

 which are thus more and more thrown into action are especially 

 those tending to carry out or to assist expiration ; and at last if 

 no relief is afforded the violent respiratory movements give way 

 to general convulsions of the whole body. By the violence of 

 these convulsions the whole nervous system becomes ex- 

 hausted, the convulsions soon cease, and death is ushered in 

 with a few infrequent and long-drawn breaths. 



It has been surmised that the excitability of the respiratory 

 nerve-centre is due to certain chemical substances which act as 

 stimulants. When the blood is rich in oxygen this substance is 

 oxidized or burned, and removed so fast that it is able to exert 

 but little influence on the respiratory nerve-centre ; when, how- 

 ever, the blood is poor in oxygen, this substance accumulates 

 and the nerve-centre is powerfully stimulated. Thus when the 

 'blood needs oxygen, the respirations are increased to get, if 

 possible, more air into the lungs ; if the blood is too rich in 

 oxygen, the respirations become abnormally quiet and shallow. 



Modified respiratory movements. Various emotions may be 

 expressed by means of the respiratory apparatus. 



Sighing is a deep and long-drawn inspiration, chiefly through 

 the nose. 



Yawning is an inspiration, deeper and longer continued than 

 a sigh, drawn through the widely open mouth, and accompanied 

 by a peculiar depression of the lower jaw. 



Hiccough is caused by a sudden, inspiratory contraction of the 



