344 MANUAL OF PHYSIOLOGY. 



infundibuli. It varies much with the force of respiration and 

 many other circumstances. In children up to ten or twelve years 

 of age it is remarkably sharp and loud, and is called "puerile 

 breathing." 



NERVOUS MECHANISM OF KESPIRATION. 



The movements of respiration go on rhythmically without any 

 voluntary effort, and even when we are quite awake they occur 

 almost without our being conscious of them, and repeated varia- 

 tions take place in the rate, depth and general type of our respi- 

 rations without our knowledge. Indeed, if this self-regulating 

 arrangement did not exist, we should have to devote much of our 

 attention to adapting our respiratory movements to the ever- 

 changing requirements of the gas interchange of the blood. 



Like all other groups of skeletal muscles, those which act on 

 the thorax are regulated by nerves and work together in harmony. 

 These coordinated movements are so far under the control of the 

 will that any of the groups of muscles may be employed sepa- 

 rately, or in conjunction. 



But the respiratory differ from the other skeletal muscles, in 

 that they undergo rhythmical coordinated contractions which are 

 not directed by our will, and can be influenced by it only to a 

 certain extent, for they cannot be made to cease altogether. 



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 



