NERVOUS MECHANISM OF RESPIRATION. 345 



in the medulla oblongata, injury of which causes instant cessa- 

 tion of the respiratory movement. 



This vital point, or nceud vital, is situated in the floor of the 

 fourth ventricle, near the point of the calamus scriptorius, and is 

 now commonly spoken of as the respiratory centre. 



From this centre the impulses which give rise to and regulate 

 the all-important respiratory movements rhythmically, pass down 

 the spinal cord and nerves. So long as the nervous communica- 

 tion between the centre and the muscles is intact, the movements 

 go on with undisturbed regularity ; if it be cut off, or the centre 

 destroyed, they instantly stop. 



What keeps this centre active? It has been already stated 

 that all the conditions of the body which cause an increased tissue 

 change use up a greater amount of oxygen, and give off more 

 carbonic acid, therefore are accompanied by more active move- 

 ments of the respiratory muscles. From this it would appear 

 that there exists some relation between the activity of the respira- 

 tory centre and the condition of the blood a deficiency of oxygen 

 or an excess of carbonic acid gas calling forth increased action. 

 One has only to hold one's breath as long as possible, and note 

 the series of rapid and deep respirations that follow such a tem- 

 porary impediment to the proper oxygenation of the blood, in 

 order to see that an involuntary respiratory centre is profoundly 

 influenced by a deficiency of oxygen. Experimentally it can be 

 shown that the effect is produced, in a great measure at least, in 

 the medulla itself, by the blood flowing through it, and not 

 by the action of the venous blood circulating through. distant 

 organs, and reflexly affecting the centre. It has also been 

 shown that the temperature of the blood circulating through 

 the medulla changes the activity of the centre, for, if the blood 

 in the carotids be warmed, the respiratory movements become 

 more rapid. 



The respiratory centre is, then, a good example of what is 

 called an " automatic nerve centre," not depending upon nerve 

 impulses from afar for its energy, nor merely reflecting the in- 

 fluences of other centres, but acquiring its energy from the ther- 

 mal and chemical condition of the blood which flows through it, 



