CHEMICAL CORRELATION OF RESPIRATORY ACTIVITIES 367 



carotid artery of the one animal supplies the brain of the other, then 

 the prevention of effective respiration in the animal of which the 

 brain is receiving normal blood produces no hyperpnea or dyspnea, 

 while the other animal, which can breathe freely, but whose brain is 

 supplied with blood from an asphyxiated animal, shows every sign of 

 respiratory distress. 



Two possibilities evidently exist, therefore. Either the stimulation 

 of the respiratory center which results from prevention of the normal 

 ventilation of the lung is due to a lack of sufficient oxygen in the blood 

 which supplies the respiratory center, or else it is attributable to the 

 accumulation of carbon dioxide in the blood-supply. It may be that 

 both of these factors play some part in determining the total result, 1 

 but by far the predominant part is that which is played by the accu- 

 mulation of Carbon Dioxide, as will be clear from the following con- 

 siderations : 



In the first place it has long been known that a very slight increase, 

 relatively speaking, in the carbon-dioxide content of the inspired air 

 leads to a considerable acceleration and increase in amplitude of the 

 respiratory movements. To bring about a like increase, by a mere 

 decrease of oxygen, provided thorough ventilation of the lungs be 

 secured by unobstructed breathing movements, requires a very much 

 greater diminution of oxygen pressure than the requisite increase of 

 carbon-dioxide pressure. Then, again, in the performance of muscular 

 work there is little or no deficiency of oxygen in the blood, but the 

 content of carbon dioxide must be increased, for the output of carbon 

 dioxide in the lungs is increased and, furthermore, the carbon-dioxjde 

 content of the air contained in the alveoli of the lungs, the Alveolar 

 Air, is increased by work. The performance of work being in fact 

 accomplished by means of the liberation of energy derived from oxi- 

 dations, the end-products of these oxidations, among which carbon 

 dioxide and water are predominant, must accumulate in the tissues 

 during the performance of work, and therefore be more abundantly 

 contained in the blood than during rest. The supply of oxygen to the 

 tissues, on the other hand, is normally superabundant, and the muscular 

 tissues, moreover, contain a certain reserve-store of oxygen, and when 

 they are excised from the body, they can contract and perform work for 

 a considerable period in an atmosphere which is devoid of oxygen. 

 In fact, the saturation of the arterial Hemoglobin with oxygen is so 

 nearly complete in normal respiration that the hyperpnea which 

 results from energetic exercise would be devoid of utility if its object 

 were the introduction of more oxygen to the tissues. Finally, we 

 have seen that Apnea may result from enhanced ventilation of the 

 lungs, but this is not due to an increased intake of oxygen or of satur- 

 ation of the tissues therewith, for it may be brought about as well by 



1 It is improbable that lack of oxygen is in itself a stimulus to the respiratory center 

 or any other tissue. The apparent stimulation, if it occurs at all, is only indirectly 

 attributable to deficiency of oxygen. 



