362 RESPIRATION [CH. XXIV. 



The evidence in favour of the automatic activity of the centre is 

 the following : 



(1.) 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 

 alae 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 out impulses to them. If the two vagus 

 nerves are cut, these movements continue ; this shows that afferent 

 impulses from the vagus are not essential. As the blood gets more 

 and more venous, the movements become more pronounced. The 

 question has arisen 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 increase in the carbonic acid 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 stimulation of the 

 centre by venous blood. It is not due (or not wholly due) to the 

 action of the venous blood on the terminations of the vagi in the 

 lungs, as the same phenomenon occurs when these nerves are cut ; and, 

 moreover, dyspnoea takes place if the venous blood is allowed to 

 circulate through the brain alone, and not through the lungs at all. 

 For instance, it ensues when localised venosity of the blood is produced 

 in the brain by ligature of the carotid and vertebral arteries. 



But, as before stated, the normal activity of the respiratory centre 

 is not automatic, it is reflex, and the principal afferent channel is the. 

 vagus. The way in which it works has been made out of recent years 

 by Marckwald, Bering, and Head. The following is a brief rteumt of 

 Head's results : 



His method of recording the movements was by means of that con- 

 venient slip of the diaphragm which is found in rabbits (see p. 356). 



His method of dividing the vagus was by freezing it ; he laid it 

 across a copper wire, the end of which was placed in a freezing 

 mixture. This method is free from the disadvantage which a cut 

 with a knife or scissors possesses, namely, a stimulation at the 

 moment of section. On dividing one vagus, respiration became 

 slightly slower and deeper ; on dividing the second nerve, this effect 

 was much more marked. 



On exciting the central end of the divided nerve, inspiratory 

 efforts increased until at last the diaphragm came to a standstill in 

 the inspiratory position. But if a weak stimulus was employed, the 

 reverse was the case ; the expiratory efforts increased, inspiration 

 becoming weaker and weaker, until at last the diaphragm stopped in 



