RESPIRATION. 199 



Methods of Stimulation of Respiratory Centre. The means by 

 which the respiratory centre or centres are stimulated must now be con- 

 sidered. 



It is well known that the more venous the blood, the more marked 

 are the insprator^ impulses, and that if the air is prevented from enter- 

 ing the chest, that the respiration in a short time becomes very labored. 

 The obstruction to the entrance of air, whether partial or complete, is 

 followed by an abnormal rapidity of the inspiratory acts, which make up 

 even in depth for the previous stoppage. The condition caused by the 

 obstruction, or by any circumstance in consequence of which the oxygen 

 of the blood is used up in an abnormally quick manner, is known as 

 dyspnwa, and as the aeration of the blood becomes more and more inter- 

 fered with, not only are the ordinary respiratory muscles employed, but 

 also those extraordinary muscles which have been previously enumerated 

 (p. 181). As the blood becomes more and more venous the action of the 

 medullary centre becomes more and more active. The question arises as 

 to what quality of the venous blood it is which causes this increased ac- 

 tivity ; whether it is its deficiency of oxygen or its excess of carbonic 

 acid. This question has been answered by the experiments, which show 

 on the one hand that dyspnoea occurs when there is no obstruction to the 

 exit of carbonic acid, as when an animal is placed in an atmosphere of 

 nitrogen, and that it cannot therefore be due to the accumulation of car- 

 bonic acid ; and on the other, that if plenty of oxygen is supplied, true 

 dvspnoea does not occur, although the carbonic acid of the blood is in ex- 

 cess. It is highly probable, therefore, that the respiratory centre is 

 stimulated to action by the absence of sufficient oxygen in the blood cir- 

 culating in it, and not by the presence of an excess of carbonic acid. 



The means by which the vagus is excited to increase the activity of 

 the respiratory centre, appears to be that the venous blood circulating 

 in the lungs, or the air in the pulmonary alveoli, stimulates the peri- 

 pheral fibres of the nerve. If these be the stimuli it will be evident that 

 the vagus action must help to increase the activity of the centre, when 

 the blood in the lungs becomes more and more venous. No doubt the 

 venous condition of the blood affects all the sensory nerves in a similar 

 manner. It has been shown that the circulation of too little blood 

 through the centre, as when its blood supply is cut off, greatly increases 

 its inspiratory action. 



Effects of Vitiated Air. Ventilation. As the air expired from 

 the lungs contains a large proportion of carbonic acid and a minute 

 amount of organic putrescible matter, it is obvious that if the same 

 air be breathed again and again, the proportion of carbonic acid and 

 organic matter will constantly increase till it becomes unfit to be breathed, 

 but long before this point is reached, uneasy sensations occur, such as 

 headache, languor, and a sense of oppression. It is a remarkable fact, 



