RESPIRATORY INTERCHANGE UNDER DIFFERENT CONDITIONS 525 



exposing an animal to a high temperature or by heating its blood 

 directly as it traverses the carotid artery.^ These types of hyperpnea, 

 however, are not dependent upon the gaseous composition of the blood 

 and should, therefore, be classified as ordinary reflex reactions. 



Dyspnea. — If prolonged, the condition of hyperpnea gradually 

 passes over into the condition of dyspnea, the essential characteristic 

 of which is labored breathing. Its cause lies either in a deficiency of 

 oxygen or in an excess of carbon dioxid; most generally, however, 

 these two factors act in unison. In accordance with this statement, 

 it must be evident that an animal may be rendered dyspneic in two 

 ways, viz., by interfering with its respiratory activity in a mechanical 

 way or by altering the composition of the inspired air. Among the 

 former occurrences might be mentioned the partial occlusion of the 

 respiratory passage by foreign bodies or by pressure from without. 

 In a chemical way, dyspnea may be produced either by lessening the 

 tension of the oxygen or by increasing the tension of the carbon dioxid. 

 The former is designated as O = dyspnea, and the latter as CO2 = 

 dyspnea. An animal may also be rendered dyspneic by permitting 

 it to breathe an indifferent gas, such as pure nitrogen or hydrogen. 

 Curiously enough, the dyspnea then ensuing cannot be prevented by 

 lessening the carbon dioxid tension of the blood, which would natu- 

 rally diminish the excitation of the respiratory center. It is also 

 possible to render an animal dyspneic by permitting it to inhale an 

 increased amount of carbon dioxid. In this case, the oxygen cannot 

 be the deciding factor, because the occurrence of this dyspnea cannot 

 be prevented by simultaneously raising the tension of this gas. De- 

 ficiencies in oxygen, which finally give rise to dyspnea, may be pro- 

 duced by bleeding, by the fixation of the hemoglobin by carbon mon- 

 oxid, by hemolysis of the red corpuscles, and by any impairment of 

 the cardiovascular system tending to lessen the vascularity of the 

 tissues. 



While the general picture of dyspnea always remains the same, 

 certain differences may nevertheless be noted which allow us to differ- 

 entiate the = dyspnea from the €62 = dyspnea. The former 

 usually runs a longer course and finally leads to marked motor disturb- 

 ances. The latter, on the other hand, immediately assumes a more 

 depressive and more narcotizing character. Moreover, during the 

 former the respirations are prone to be rather frequent and display a 

 forced inspiratory character, whereas during the latter they are slow 

 and of a pronounced expiratory type. 



Asphyxia. — This condition represents the final state of dyspnea, a 

 state of functional exhaustion and collapse. It signifies that the 

 deprivation of oxygen has been completed. The powerful respiratory 

 movements ordinarily observed during the later stages of dyspnea, 



^ Fick and Goldstein, Verhandl. math.-naturw. Ges., Wiirzburg, ii, 156. The 

 term polypnea has been applied to this form of hyperpnea by Richet, Compt. 

 rend., xcix, 1884, 279. 



