340 MANUAL OF PHYSIOLOGY. 



In short, the rhythmical coordinated movements of respiration 

 are not only brought about, but are also regulated by an invol- 

 untary nervous mechanism. Since we are unconscious of its 

 action, it certainly is not dependent on the voluntary centres. 

 Moreover, we know that the upper parts of the brain are not 

 needed for regular breathing, because animals born with deficient 

 development of cranium and brain can breathe quite rhythmi- 

 cally ; and removal of the brain of birds, etc., causes no inter- 

 ruption of the respiratory movements. We know, however, that 

 an injury to the upper part of the spinal cord causes death by 

 stopping respiration. The regulating centre must then be lower 

 than the cerebral centres, and higher than the cervical part of 

 the spinal marrow. The direct evidence of the seat of this centre 

 was found by Flourens, who showed that a localized spot exists 

 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 



