386 RESPIRATION. 



arterial blood is calculated from the percentage of carbon monoxid 

 breathed by the subject of the experiment, and from the final 

 saturation of his blood with carbon monoxid " (Pembrey). Their 

 results are 26.2 per cent, of an atmosphere, or 200 rum. of mercury, 

 about twice that of oxygen in the alveoli, which would confirm 

 Bohr's views that diffusion cannot account for the absorption of 

 oxygen by the blood while flowing through the pulmonary cap- 

 illaries, but that it is to be attributed to the epithelial cells of the 

 alveoli. 



Notwithstanding these results, which need further investiga- 

 tion, diffusion is usually regarded as the principal factor in de- 

 termining the gaseous interchanges between the air and the blood. 



Causes of the Interchange , of O and CO 2 between 

 the Blood and the Tissues. This process constitutes in- 

 ternal respiration. 



Oxygen when it reaches the tissues by the blood is immediately 

 taken up by them and enters into chemical combination, so that 

 as oxygen it may be said not to exist, except momentarily. On 

 the other hand, the tension of oxygen in arterial blood is rela- 

 tively high, so that its passage from the blood to the tissues is 

 readily accounted for. The tension of CO 2 in the tissues is about 

 58 mm. of mercury higher than in the blood ; hence its passage 

 outward from the tissues to the blood. 



Innervation of the Respiratory Apparatus. The nerv- 

 ous supply to the respiratory apparatus comes from the respiratory 

 center, a collection of nerve-cells in the lower part of the medulla 

 oblongata, though its exact location is not yet determined. Other 

 centers, subsidiary centers, have been described, but their inde- 

 pendence of the principal center is questioned. 



The respiratory center is in reality made up of two centers, 

 one for each side, so that, although anatomically connected and 

 ordinarily acting together, yet if one center is broken up, while 

 the respiratory movements on that side cease, those on the other 

 side continue. 



Besides this double character of the center, each half is made 

 up of an inspiratory and an expiratory center i. <?., the nervous 

 impulses which originate and pass out from the inspiratory center 

 produce inspiratory movements, and those from the expiratory 

 center bring about movements of expiration. 



The respiratory center is both an automatic and a reflex center 

 L e.j it sends out spontaneously impulses which result in move- 

 ments of the respiratory muscles, constituting its automatism ; and 

 it may also be excited reflexly i. e. 9 by impulses reaching it from 

 without. Its reflex character is most marked, and it is doubtless 

 as a reflex center that its function is ordinarily performed. 



Rhythm of the Respiratory Movements. One of the 

 striking characteristics of the respiratory movements is their 



