THE RESPIRATORY SYSTEM. 395 



by reports to the responsible center in the medulla, and if the 

 medulla is a sharer in the lack, as it naturally would he, it takes 

 action independently. One of the most ohvious instances in 

 which there is oxygen starvation is when there is hindrance to 

 the entrance of air, owing to obstruction in the respiratory tract. 



At first the breathing is merely accelerated, with perhaps 

 some increase in the depth of the inspirations (hyperpnoBO), a 

 stage which is soon succeeded by labored breathing (dyspnoea), 

 which, after the medulla has called all the muscles usually em- 

 ployed in respiration into violent action, passes into convul- 

 sions, in which every muscle may take part. 



In other words, the respiratory impulses not only pass along 

 their usual paths as energetically as possible, but radiate into 

 unusual ones and pass by nerves not commonly thus set into 

 functional activity. 



It would be more correct, perhaps, to assume that the vari- 

 ous parts of the nervous system are so linked together that ex- 

 cessive activity of one set of connections acts like a stimulus to 

 rouse another set into action, the order in which this happens 

 depending on the law of habit — habit personal and especially 

 ancestral. An opposite condition to that described, known as 

 apnoe,a, may be induced by pumping air into an amimal's chest 

 very rapidly by a bellows ; or in one's self by a succession of 

 rapid, deep respirations. 



After ceasing, the breathing may be entirely interrupted 

 for a brief interval, then commence very quietly, gradually in- 

 creasing to the normal. 



Apnoea has been interpreted in two ways. Some think that 

 it is due to fatigue of the muscles of respiration or the respira- 

 tory center; others that the blood has under these circum- 

 stances an excess of oxygen, which so influences the respiratory 

 center that it is quieted (inhibited) for a time. 



The latter view is that usually adopted ; but considering that 

 apnoea results from the sobbing of children following a pro- 

 longed fit of crying, also in Cheyne-Stokes and other abnormal 

 forms of breathing, and that the blood is normally almost satu- 

 rated with oxygen, it will be agreed that there is a good deal to 

 be said for the first view, especially that part of it which repre- 

 sents the cessation of breathing as owing to excessive activity 

 and exhaustion of the respiratory center. We find such a calm 

 in asphyxia after the convulsive storm. Perhaps if we regard 

 the respiratory center as double, half being situated on each side 



