274 RESPIRA TION 



the other hand, the absorption of oxygen in the lungs is not seriously 

 interfered with, because the available alveolar partial pressure of 

 oxygen is so high (100 mm. or more) that it can be considerably 

 diminished without causing any appreciable diminution in the 

 percentage saturation. 



There is evidence that in dialyzed haemoglobin solutions the 

 haemoglobin is all in the form of single molecules. The addition of 

 salts as they exist in blood produces a certain amount of aggrega- 

 tion or clumping of the molecules, and as already pointed out, this 

 alters the shape of the dissociation curve. The S-shape of the 

 curve of blood indicates that blood will part with its oxygen at 

 low oxygen pressures, as in passing through the tissues, much more 

 readily than a corresponding solution of salt -free oxyhaemoglobin ; 

 and that it will take up oxygen more easily at high pressures, as 

 in passing through the lungs. As regards the effect of acids, it 

 was observed by Bohr (p. 255) that an increase in the carbon dioxide 

 tension of blood diminishes its combining power for oxygen, and 

 therefore favours the giving up of oxygen to the lymph and tissues. 

 This may have an important influence on internal respiration. The 

 effect is much more marked where the oxygen tension is low than 

 where it is high, so that in the lungs the taking up of oxygen is 

 scarcely interfered with even by a high carbon dioxide tension. 



According to Barcroft, the combined effect of the increased tem- 

 perature, the salts and the carbon dioxide of blood (at the partial 

 pressure of 40 mm. of mercury) is to make possible a diffusion of 

 oxygen into the tissues at 100 times the speed at which it would 

 diffuse from a salt-free solution of oxyhaemoglobin at a tempera- 

 ture of 16 C. This astonishing feat is accomplished without 

 sensibly decreasing the percentage saturation with oxygen of the 

 blood passing through the lungs. 



SECTION VI. RELATION OF RESPIRATION TO THE NERVOUS 



SYSTEM. 



The Respiratory Centre and its Connections. Unlike the beat of 

 the heart, the respiratory movements are entirely dependent on the 

 central nervous system. The ' centre ' which presides over them is 

 situated in the spinal bulb. It is a bilateral centre that is, it has 

 two functionally symmetrical halves, one on each side of the middle 

 line. Each of these halves has to do more particularly with the 

 respiratory muscles of its own side, for destruction of one-half of 

 the spinal bulb causes paralysis of respiration only on that side. 

 Anatomically the respiratory centre has not been sharply localized, 

 but it lies lower than the vaso-motor centre, not far from the point 

 of the calamus scriptorius. Stimulation of this region during apncea 

 (p. 283) is stated to cause co-ordinated inspiratory movements and 



