RESPIRATION. 551 



umes per cent, of CO 2 , dyspnoea occurs, notwithstanding an abundance of O 

 (p. 544) both in the air and in the blood ; indeed, the quantity of O in the 

 blood may be above the normal. Fredericq l in ingenious experiments has 

 directly demonstrated the influence of the quantity of CO 2 in the blood upon 

 the respiratory movements. He took two rabbits or dogs, A and B, ligated the 

 vertebral arteries in each, exposed the carotids, and ligated one in each animal. 

 The other carotid in each was cut, and the peripheral end of the vessel of one 

 was connected by means of a cannula with the central end of the vessel of the 

 other, so that the blood of animal A supplied the head (respiratory centre) of 

 animal B, and vice versd. When the trachea of animal A was ligated or com- 

 pressed the animal B showed signs of dyspnoea, because its respiratory centre 

 was now supplied with the venous blood from A. On the contrary, animal A 

 exhibited quiet respirations, almost apnoeic, because its centre received the 

 thoroughly arterialized blood from B, in which the respiratory movements were 

 augmented. In a second series of experiments blood was transfused through 

 the head : when the blood was laden with CO 2 marked dyspnoea resulted ; 

 when arterial blood was transfused the normal respirations were restored. 



While dyspnoea may be caused by the respiration of an atmosphere either 

 deficient in O (" O-dyspnoea ") or containing an excess of CO 2 (" CO 2 -dysp- 

 ncea "), the phenomena in the two cases are in certain respects different : When 

 an animal breathes pure N, thus causing O-dyspncea, the dyspnoea is character- 

 ized especially by frequent respiratory movements with vigorous inspirations, 

 whereas if the atmosphere be rich in O and contain an excess of CO 2 the 

 respirations are especially marked by a slower rate and by the depth and vigor 

 of the expirations ; O-dyspnoea continues for a long time before death ensues, 

 and is more severe ; in O-dyspnoea the absorption of O is diminished, but the 

 excretion of CO 2 is practically unaffected ; in O-dyspncea the attendant rise of 

 blood-pressure (p. 555) is more marked and lasting ; in O-dyspncea death is 

 preceded by violent motor disturbances which are absent in CO 2 -dyspnoea. 

 Blood poor in O (O-dyspncea) affects chiefly the inspiratory portion of the 

 respiratory centre (p. 565), while blood rich in CO 2 (CO 2 -dyspncea) affects 

 chiefly the expiratory portion ; hence in the former the dyspnoea is manifest 

 especially in an increase in the frequency of the respirations (hyperpnoea) and 

 in the vigor of the inspirations, while in the latter it is manifest in a lessened 

 rate, strong expirations, and expiratory pauses. 



The marked increase in the depth of the respiratory movements in CO 2 - 

 dyspnoea is not solely due to the direct action of CO 2 upon the respiratory 

 centre, for Gad and Zagari 2 have shown that CO 2 in abundance in inspired air 

 acts upon the terminations of the sensory nerves of the larger bronchi and 

 thus reflexly excites the respiratory centre. In a research on dogs these ob- 

 servers opened the trachea and passed glass tubes through the trachea and the 

 larger bronchi to the smaller bronchi. Before the tubes were inserted the 

 inhalation of CO 2 caused a considerable deepening of the respiratory move- 



1 Bull Acad. Roy. Med. Belgique, vol. 13, pp. 417-421. 

 . 2 DuBois-Reymond's Archiv f. Physiologic, 1890, p. 588. 



