486 EFFECTS OF DEFICIENCY OF OXYGEN. [BOOK n. 



the contrary, especially as exhaustion begins to set in, the 

 rhythm becomes slower out of proportion to the weakening of 

 the individual movements. 



301. The question naturally arises, Does this condition of 

 the blood affect the substance of the central nervous system, that 

 is to say, of the respiratory centre in the spinal bulb (and the 

 subsidiary spinal nervous mechanisms) directly, or does it pro- 

 duce its effect by stimulating the peripheral ends of afferent 

 nerves in various parts of the body, and, by the generation there 

 of afferent impulses, indirectly modify the action of the central 

 nervous system ? Without denying the possibility that the latter 

 mode of action may help in the matter, as regards not only the 

 vagus, but all afferent nerves, the following facts seem to shew 

 that the main effect is produced by the direct action of the blood 

 on the central nervous system and indeed on the bulbary res- 

 piratory centre itself. If the spinal cord be divided below the 

 bulb, and both vagi be cut, want of proper aeration of the blood 

 still produces an increased activity of the respiratory centre, as 

 shewn by the increased vigour of the facial respiratory move- 

 ments ; in such a case, it must act directly on the respiratory 

 centre, for all afferent paths along the nerves, except the few 

 cranial ones, have been blocked by the operation. The same 

 direct action is further shewn by the following " cross circula- 

 tion " experiment. In two animals the peripheral portion of one 

 carotid of one animal is connected by a tube with the central por- 

 tion of one carotid of the other animal, the other carotid in each 

 animal being tied. Hence the brain and the brain only of one 

 animal is supplied by the blood of the other animal, the rest of 

 its body being supplied by its own blood. If now respiration 

 be stopped in one animal the other becomes dyspnceic, while it 

 in itself shews no dyspnoea ; it is the animal to whose brain 

 (spinal bulb) alone too venous blood is brought, not the animal 

 the whole of whose body is supplied with the too venous blood, 

 which manifests disturbance of the respiratory centre. Again, 

 if in an animal the supply of blood be cut off from the spinal bulb 

 by ligature of the carotid and intervertebral arteries dyspnoea 

 is produced, though the operation produces at first no change in 

 the blood generally, but simply affects the respiratory condition 

 of the medulla itself by cutting off its blood-supply, the imme- 

 diate result of which is an accumulation of carbonic acid and a 

 paucity of available oxygen in the nervous substance of that 

 region. If the blood in the carotid artery in an animal be 

 warmed above the normal, a dyspnoea is produced which, though 

 apparently not quite identical with the dyspnoea caused by im- 

 perfect arterialization of the blood, shews that the too high 

 temperature of the blood directly affects the activity of the 

 respiratory centre. We may conclude therefore that the con- 

 dition of the blood affects respiration by acting directly on the 

 respiratory centre. 



