382 BESPIKATION. 



acting directly on the respiratory centre. Moreover, it is the medullary 

 centre which, at all events in adult animals, is affected by the too venous 

 blood, since after the division of the spinal cord below the medulla, dyspnoeic 

 thoracic respiratory movements and convulsions do not follow upon exclusion 

 of air. They are, however, stated to occur in new-born animals, indicating 

 that the subsidiary mechanisms in the upper spinal cord, of which we spoke 

 in 306, may be also affected by the too venous blood ; but the doubts which 

 we previously urged hold good in these cases also. 



While the respiratory centre is thus being affected by the too venous 

 blood, it is, until exhaustion begins to set in, more irritable, more easily and 

 largely affected by afferent impulses than in its normal condition. During 

 dyspnoea a stimulus which applied to the vagus or to some other sensory nerve 

 under normal conditions would produce little or no effect, may start very 

 powerful respiratory movements. 



315. Deficient aeration produces two effects in blood : it diminishes the 

 oxygen and increases the carbonic acid. Do both of these changes affect 

 the respiratory centre, or only one, and if so, which ? When an animal is 

 made to breathe an atmosphere containing nitrogen only, the exit of car- 

 bonic acid by diffusion is not affected, and the blood, as is proved by actual 

 analysis, contains no excess of carbonic acid. Yet all the phenomena of 

 dyspncea are present, and if the experiment be continued, convulsions ensue 

 and the animal dies in asphyxia. In this case the result can only be 

 attributed to the deficiency of oxygen. On the other hand, if an animal be 

 made to breathe an atmosphere rich in carbonic acid, but at the same time 

 containing abundance of oxygen, though the breathing becomes markedly 

 deeper and also somewhat more frequent, there is no culmination in a con- 

 vulsive asphyxia, even when the quantity of carbonic acid in the blood, as 

 shown by direct analysis, is very largely increased. On the contrary, the 

 increase in the respiratory movements may after a while pass off, the animal 

 becoming unconscious, and appearing to be suffering rather from a narcotic 

 poison than from simple dyspnoea ; the excess of carbonic acid in the blood 

 appears to affect other parts of the central nervous system, and especially 

 portions of the brain, more profoundly than it does the respiratory centre. 

 It has been maintained by some that while a deficiency of oxygen promotes 

 inspiratory movements, an excess of carbonic acid stimulates the expiratory 

 movements, the nervous mechanisms being so arranged that a lack of oxygen 

 leads to an effort to get more of it, and a too great load of carbonic acid to 

 an effort to get rid of it ; but the facts are opposed to the existence of any 

 such teleological adaptation. It is obvious, however, that a lack of oxygen 

 and an excess of carbonic acid affect the respiratory centre in very different 

 ways, and that in ordinary cases of interference with the interchange in the 

 lungs, as in deficient aeration, it is the lack of oxygen which plays the 

 principal part in developing the abnormal respiratory movements. We 

 may infer that it too is chiefly concerned in regulating the more normal 

 respiration, but cannot as yet say what is the exact share so be attributed to 

 the carbonic acid. 



We may here point out that it is not to be supposed that each breath is 

 determined by the condition of the blood flowing through the capillaries of 

 the medulla at the moment preceding that breath ; it is not to be imagined 

 that each breath is the result of the lack of oxygen felt immediately before. 

 On the contrary, as we have previously urged, the respiratory centre, like 

 the cardiac substance, is an automatic centre; the respiratory impulses issue 

 from it in rhythmic series as a result of the molecular changes of the 

 metabolism going on in its substance ; and whatever affects that rhythm, 

 whether few or many beats be influenced, produces its result by modifying 



