7^2 THE RESPIRATORY CENTER. 



of oxygen and carbon dioxid present. In this connection the following 

 distinctions are to be recognized : 



1. Apnea, that is the cessation of the respiratory movements in 

 consequence of deficient need therefor. It occurs when the blood is 

 saturated with oxygen and is deficient in carbon dioxid. Blood in. such a 

 state fails to stimulate the center, and, therefore, the muscles con- 

 trolled by it remain at rest. The fetus is in this condition; likewise 

 some animals in the state of hibernation. If, further, an abundance of 

 air is made to pass into the lungs of animals by means of artificial 

 respiratory apparatus, they cease to breathe because the marked arte- 

 rialization of their blood does not permit of stimulation of the res- 

 piratory center. If, further, a similar state of the blood is induced by 

 rapid and deep respirations apneic pauses of considerable duration occur. 



A. Ewald found the blood in the arteries of apneic animals almost completely 

 saturated with oxygen, while the amount of carbon dioxid was diminished. The 

 venous blood contained less oxygen than under normal conditions. The latter 

 fact is probably due to the circumstance that the apneic state of the blood greatly 

 reduces the blood-pressure, and in consequence .the circulation is slowed. There- 

 fore, the oxygen can be taken from the capillary blood in much larger amount. 

 In general, however, the consumption of oxygen during the state of apnea is 

 not increased. Gad calls attention to the fact that on forced artificial respiration 

 the pulmonary alveoli are greatly filled with atmospheric air, and in consequence 

 they are capable of arterializing for a considerable time the blood entering the 

 lungs, so that the necessity for respiration must be diminished. According to 

 Gad and Knoll the respiratory center during apnea is in a state of diminished 

 irritability, which can be induced reflexly through the forcible stretching of the 

 terminal pulmonary branches of the vagi in connection with the artificial respira- 

 tory movements. This is shown also by the fact that the respiratory movements 

 commence again only after the blood has already become dark, when, in conse- 

 quence of this venous state of the blood, signs of irritation through the venosity 

 of the blood appear in the heart, the vascular system and the intestine. Apnea 

 cannot be induced in young mammals. 



2. The normal stimulus for the respiratory centers for quiet breath- 

 ing, eupnea, is furnished by a state of the blood in which the amount of 

 oxygen and carbon dioxid does not exceed the normal limits. 



3. All factors that diminish the normal amount of oxygen and in- 

 crease the amount of carbon dioxid in the blood circulating through the 

 centers cause acceleration and deepening of the respiration, which finally 

 may increase to the point of strained and laborious activity of all of the 

 respiratory muscles. This condition is designated dyspnea. 



In case of normal breathing and beginning air-hunger the gases in the blood, 

 according to Gad, irritate only the inspiratory center. Expiration takes place 

 reflexly through irritation of the pulmonary branches of the vagus stimulated by 

 the distention of the lungs. Gad believes that the normal respiratory movements 

 are excited by the carbon dioxid. If dyspneic blood be passed through the 

 vessels of the brain of a normal animal, the latter becomes dyspneic. In case of 

 dyspnea in consequence of excessive physical activity, an as yet unknown body, 

 formed as a result of the muscular activity, acts, in addition to the change in 

 the gases of the blood mentioned, as an irritant for the center, perhaps as an 

 acid. The alterations in the dyspneic respiratory rhythm are described on 

 p. 211. 



4. If the abnormal conditions of the blood mentioned contiriue to 

 exert an irritant effect, or if they are further intensified, there finally 

 results a state of exhaustion in consequence of overstimulation of the 

 respiratory centers ; the respiration is again diminished with respect to 

 the number and the depth of the movements, and later on only a few 



