278 ESSENTIALS OF PHYSIOLOGY. 



breaths become smaller and finally stop. During the next apnoeic 

 pause, the blood again becomes deficient in oxygen, and a fresh forma- 

 tion of lactic acid takes place in the respiratory centre. 



When this form of breathing occurs in disease, it can be removed 

 either by allowing the patient to breathe nearly pure oxygen, which 

 improves the nutrition of the respiratory centre and prevents the 

 formation of lactic acid, or by the administration of air containing a 

 slight excess of carbonic acid, which, by raising the tension of this gas 



miniiiiniiiiiii 



FIG. 112. Cheyne- Stokes respiration. (Pembrey and Allen.) From Practical 

 Physiology, by Pembrey and others. 



in the alveolar air and blood, increases the strength of the stimulus 

 to the respiratory centre. 



SECTION IV. 

 TISSUE RESPIRATION. 



The processes of external respiration, which have been thus far 

 considered, are so adapted that the blood, when it reaches the capillaries, 

 is almost fully saturated with oxygen. More important, however, is 

 tissue respiration, which consists in the transference of oxygen from the 

 blood to the tissues and the return of carbonic acid from the tissues to 

 the blood. 



Oxygen passes from the blood to the tissues by diffusion, and the 

 amount which is available for the tissues depends largely upon the 

 extent to which oxygen is set free in the plasma by the dissociation of 

 oxyhsemoglobin. If the tension in the tissues were zero, as was formerly 

 supposed, the conditions during the passage of blood through the 

 capillaries would be the same as if it were exposed to a vacuum, and 

 the dissociation of oxyhsemoglobin would be almost complete. Recent 

 observations have shown, however, that the actual tension of oxygen 

 in the tissues varies from 10 to 40 mm. Hg, being highest in the glandular 



