RESPIRATION. 65 



contents is changed at each inspiration. As long as the residuary 

 air contains any oxygen, aeration of the blood will take place, pro- 

 vided always the heart continues to act ; but as soon as the contained 

 oxygen is consumed, asphyxia begins to occur, unless a fresh supply 

 is obtained. The residuary air will probably support life about three 

 minutes. 



Influence of the nerves on respiration. — The source of the ner- 

 vous influence on which the different respiratory movements depend, 

 is one and the same, although the nerves implicated in these move- 

 ments are very various. The medulla oblongata is the source from 

 which the nervous influence for the respiratory motions is derived, 

 and the spinal cord is, as it were, the trunk of the nerves which 

 arise irom it. If the spinal cord is divided above the point where 

 the dorsal nerves are given off, the motions of the ribs and abdo- 

 minal muscles are paralysed, while the other respiratory movements 

 continue. If it is cut above the origin of the phrenic nerve, then 

 the diaphragm is paralysed, while the nerves given ofl* from the 

 medulla oblongata itself still continue to exert their function. In- 

 jury of the medulla oblongata arrests instantly all the respiratory 

 movements, both those dependent on the par vagum, and those on 

 the spinal nerves. 



The respiratory movements are partly voluntary, partly invo- 

 luntary. Partly voluntary, in order that they may be inservient to 

 the production of vocal sounds, and to the actions of speech, sing- 

 ing, &c. Partly involuntary, lest in sleep, or in moments of forget- 

 fulness, the movements of respiration should be suspended, and fatal 

 results ensue. 



The combined action of the respiratory muscles is under the in- 

 fluence of that portion of the nervous system called the reflex ; a 

 part which does not involve the will, or even sensation, and which 

 may continue to transmit its influence when all the other parts of 

 the nervous centres have been removed. The principal exciter, or 

 afferent nerves, are the par vagum, and the sensory branches of 

 the fifth pair, the former conveying impressions from the lungs, the 

 latter from the general surface. If the par vagum be divided on 

 both sides, the movements of respiration are greatly diminished in 

 frequency. 



Chemical phenomeyia. — The prominent phenomena in respiration 

 are, the removal of a certain quantity of oxygen from the air, and 

 its replacement by carbonic acid ; and the change in colour of the 

 blood from a dark venous hue, to a bright scarlet or arterial. The 

 relative proportions of oxygen inhaled and of carbonic acid exhaled, 

 are to each other inversely as the square roots of their specific gra- 

 vities ; that is, the quantity of oxygen absorbed will exceed that of 

 carbonic acid given off in the proportion of 1174 to 1000. Carbonic 

 acid contains precisely its own volume of oxygen ; consequently, of 



6* 



