RESPIRATION. 203 



part of the centre, producing an expiration, we must look upon the main 

 trunk of" the vagus as aiding the inspiratory, and of the superior laryngeal 

 as aiding the expiratory part of the centre, the first nerve possibly in- 

 hibiting the action of the expiratory centre, whilst it aids the inspiratory, 

 and the latter nerve having the very opposite effect. But inasmuch as 

 the respiration is slowed on division of the vagi, and not quickened or 

 affected manifestly on simple division of the superior laryngeal, it must 

 be supposed that the vagi fibres are always in action, whereas the superior 

 laryngeal fibres are not. 



It appears, however, that there are, in some animals at all events, 

 subordinate centres in the spinal cord which are able, under certain con- 

 ditions, to discharge the function of the chief medullary centre. 



The centre in the medulla may be influenced not only by afferent im- 

 pulses proceeding along the vagus and laryngeal nerves but also by those 

 proceeding from the cerebrum,, as well as by impressions made upon the 

 nerves of the skin, or upon part of the fifth nerve distributed to the nasal 

 mucous membrane, or upon other sensory nerves, as is exemplified by 

 the deep inspiration which follows the application of cold to the surface 

 of the skin, and by the sneezing which follows the slightest irritation of 

 the nasal mucous membrane. 



At the time of birth, the separation of the placenta, and the conse- 

 quent non-oxygenation of the foetal blood, are the circumstances which 

 immediately lead to the issue of automatic impulses to action from the 

 respiratory centre in the medulla oblongata. But the quickened action 

 which ensues on the application of cold air or water, or other sudden 

 stimulus, to the skin, shows well the intimate connection which exists 

 between this centre and other parts which are not ordinarily connected 

 with the function of respiration. 



Methods of Stimulation of the Respiratory Centre. It is now 



necessary to consider the method by which the centre or centres are stim- 

 ulated themselves, as well as the manner in which the afferent vagi 

 impulses are produced. 



The more venous the blood, the more marked are the inspiratory im- 

 pulses, and if the air is prevented from entering the chest, in a short time 

 the respiration becomes very labored. Its cessation is followed by an 

 abnormal rapidity of the inspiratory acts, which make up even in depth 

 for the previous stoppage. The condition caused by obstruction to the 

 entrance of air, or by any circumstance by which the oxygen of the blood 

 is used up in an abnormally quick manner, is known as dyspnaa, and as 

 the aeration of the blood becomes more and more interfered with, not 

 only are the ordinary respiratory muscles employed, but also those extra- 

 ordinary muscles which have been previously enumerated (p. 186), so that 

 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 



