RESPIRATION-. 285 



secondly, that if the vagi are divided, the respirations,, although altered 

 in character, are regular, but that if then the brain is separated from 

 the medulla, the same respiratory spasms occur. From these experiments 

 it is concluded that the automatic action of the centres consists in the 

 liberation of respiratory spasms only, and not of regular rhythmic move- 

 ments; but that impressions reaching the centres either from the cere- 

 brum or through the vagi, prevent the gathering tension in the centres 

 from becoming too great, and convert the spasms which would other- 

 wise arise into regular movements. The chief difference between the 

 action of the vagi and of the cerebral tracts, is that the former are always 

 in action, whilst the latter are not. When the vagi are in action and the 

 higher centres are not, periodic respiration takes place, that is to say, 

 respirations occurring in groups, each such group being followed by a 

 pause; a type of respiration known as Cheyne-Stokes breathing, to which 

 we shall return presently. It will be thus seen that even the ordinary 

 action of the respiratory centres is to a large extent reflex, and depend- 

 ent upon vagus or cerebral stimulation. 



Method of Stimulation of the Respiratory Centres. Apart then from 

 afferent impulses, the respiratory centres are capable of working auto- 

 matically, and this fact has been explained by the supposition that they 

 are stimulated to action by the condition of the blood circulating through 

 them, since when the blood becomes more and more venous the action 

 of the centres becomes more and more energetic, and if the air is pre- 

 vented from entering the chest, the respiration in a short time becomes 

 very labored. Any obstruction to the entrance of air indeed, whether 

 partial or complete, is followed by an abnormal rapidity of the inspira- 

 tory acts. The condition caused by any interference with the free ex- 

 change of gases in the lungs, or by any circumstance in consequence of 

 which the oxygen of the blood is used up in an abnormally quick man- 

 ner, is known as dyspnoea. If the aeration of the blood is much inter- 

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

 also those muscles of extraordinary inspiration and expiration which 

 have been previously enumerated. Thus as the blood becomes more and 

 more venous, the action of the medullary centres becomes more and 

 more active. The question has been much debated as to what quality 

 of the venous blood it is which causes this increased activity; whether 

 it is its deficiency of oxygen or its excess of carbonic acid. It has been 

 answered to some extent 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 carbonic 

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

 does not occur, although the carbonic acid of the blood is in excess. It 

 is highly probable, therefore, that the respiratory centres may be stimu- 



