RESPIRATION 207 



ligature of the common carotid. In this case the 

 respiration stopped from stimulation of the inhibitory 

 fibres in the superior laryngeal, and death resulted. 



2. In addition to its susceptibility to nervous influences, 

 the respiratory centre responds with marvellous delicacy 

 to variations in the composition of the blood which 

 circulates through it. Should an excess of venosity in 

 the blood indicate the necessity for a fuller and freer 

 interchange between the blood and the air, the respi- 

 ratory movements are at once increased in frequency 

 and depth. This is the prime cause of the exaggerated 

 breathing of dyspnoea. Experiments tend to show that 

 even the chemical products of muscular fatigue can 

 exercise this effect, and that it is by this means that 

 deeper breathing and a fuller supply of oxygen are 

 ensured during exercise. If, too, the blood which 

 reaches the centre is unduly warm, the activity of the 

 latter is increased. This may explain in part the rapid 

 respiration of fever. Certain drugs have a similar 

 influence, and strychnine is constantly used in clinical 

 medicine in order to produce such effects. On the other 

 hand, some agents, such as alcohol and opium, ulti- 

 mately depress the centre, and may end by paralyzing 

 it, and so lead to death. 



Whether the nervous or chemical method of stimu- 

 lation plays the greater part in determining the normal 

 quiet rhythmic action of the respiratory centre is not 

 yet determined, but the trend of recent physiological 

 opinion is in favour of attaching more importance to 

 the effect of the condition of the blood and less to 

 nervous influences than was the case a few years ago; 



