652 EXPERIMENTAL INVESTIGATION OF THE ACTION OF MEDICINES. 



it must constantly circulate through the pulmonary capillaries. 

 But the air in the lungs would soon become saturated with 

 carbonic acid and deprived of its oxygen by the blood which 

 comes into contact with it, and all farther diffusion would be 

 arrested, unless it also were constantly renewed. This is 

 effected by the respiratory movements. These consist in the 

 alternate enlargement and diminution of the thoracic cavity, 

 after the fashion of a pair of bellows, by the motion of its 

 walls and of the diaphragm. These movements are kept up 

 in a rhythmical manner by a nervous centre, situated in the 

 medulla oblongata and upper part of the spinal cord, which 

 sends off periodic motor impulses to the diaphragm and 

 inspiratory muscles. When the breathing is quiet, expiration 

 is usually a passive act performed by the elasticity of the 

 lungs and ribs, and by the weight of the thoracic walls. But 

 when it becomes excited, active expiratory muscles are brought 

 into action, and they receive their motor impulses from the 

 respiratory centre alternately with those of inspiration. It 

 is probable, though not certain, that the respiratory centre 

 is not a mere reflex apparatus, which simply transmits im- 

 pressions which it receives from sensory nerves to motor ones ; 

 for its activity continues, although its connections with 

 sensory nerves be almost entirely destroyed. The periodic 

 impulses which it imparts to the motor nerves of the respira- 

 tory muscles are not due to its being periodically excited by 

 impressions from afferent nerves, but to its being constantly 

 excited by the venosity of the blood circulating in it ; while 

 some resistance within itself prevents the excitation from 

 being constantly transmitted to motor nerves, and only allows 

 it to be so at periodical intervals. 



The venosity of the blood consists in the absence of oxygen 

 and the presence of carbonic acid, and it is not certain whether 

 or not both of these act as excitants to the respiratory centre ; 

 but it seems not improbable that the presence of carbonic 

 acid is the excitant, while the amount of oxygen simply alters 

 the excitability of the centre. According to this view, when 

 there is much oxygen in the blood, the excitability of the 

 centre will be slight, and little affected by any irritant applied 



