35 8 RESPIRATION. [CH. xxiv. 



of its terminations in the mucous membrane of the larynx, as 

 when a crumb is " swallowed the wrong way," produces inhibition 

 of inspiratory and increase of expiratory efforts, culminating in 

 coughing. 



These nerves, however, are none of them in constant action as 

 the vagi are, and the influence of the vagus is somewhat compli- 

 cated. Still, respiration continues after the vagi are cut. The 

 character of the respiration becomes altered, especially if both 

 nerves are severed ; it is slower and deeper. This is due to 

 the cessation of the impulses that normally run up the vagi 

 to the respiratory centre. The animal, however, lives a con- 

 siderable time ; a warm-blooded animal usually dies after about 

 a week or ten days from vagus pneumonia, due to the removal 

 of trophic influences from the lungs. Cold-blooded animals 

 live longer ; they exhibit fatty degeneration of the heart-muscle 

 also. 



The question has been much debated whether the activity of 

 the respiratory centre is automatic or reflex ; that is to say, 

 whether the rhythmic discharges proceeding from it depend 

 on local changes induced by the condition of its blood supply, 

 or on the repeated stimulations it receives by afferent nerves. 



There appears every reason to believe that the centre has the 

 power of automatism, but this is never excited under normal 

 circumstances. Normally the respiratory process is a series of 

 reflex actions. 



The evidence in favour of the automatic activity of the centre 

 is the following : 



(i.) If the spinal cord is cut just below the bulb, respiration 

 ceases, except in the case of the facial and laryngeal muscles, 

 which are supplied by nerves that originate above the point of 

 injury. The alse nasi work vigorously. Such respiration is not 

 effective in drawing any air into the chest, and so the animal 

 soon dies ; but the forcible efforts of these muscles show that the 

 respiratory centre is in a state of activity sending oxit impulses to 

 them. If the two vagus nerves are cut, these movements con- 

 tinue ; this shows that afferent impulses from the vagus are not 

 essential. As the blood gets more and more venous, the move- 

 ments become more pronounced. The question has been much 

 debated whether this increased activity of the respiratory centre 

 is due to increase of carbonic acid, or decrease of oxygen in the 

 blood which it receives. The balance of evidence shows that 

 the diminution in the oxygen is the more important of the two. 



(2.) In asphyxia, one always gets great increase of respiratory 

 activity, called dyspnoea ; this is produced by the stimulating of 



