518 RESPIRATION 



and to impart to them a greater penetrating power. In this way, at least a partial 

 aeration of the tissues may be retained for some time after the respiratory move- 

 ments have become inadequate, or after the alveolar spaces have become blocked 

 by exudated material (pneumonia). Especially beneficial results are obtained 

 in certain heart diseases, m which the supply of oxygen has become insufficient on 

 account of the impairment of the circulation. The oxygen seems to exert a stimu- 

 latmg influence upon the musculature of the heart m consequence of which its 

 contractions become more forceful. 



If the oxygen is inhaled under a pressure of from 3 to 4 atmospheres, it acts as 

 a poison, death resulting in the course of a short time from respiratory depression, 

 loss of heat and a general intoxication. The same effect may be produced by the 

 continued inhalation of ordinary air under a pressure of from 15 to 20 atmospheres. 

 It has also been shown that the development of the eggs of insects is greatly re- 

 tarded if exposed to an oxygen pressure of less than one atmosphere. Fish are 

 killed when the oxygen content of the water is raised so that 100 c.c. contain more 

 than 10 volumes of dissolved oxygen. Quite similarly, it was found by Smith ^ 

 that mice which had been exposed for several hours to an oxygen pressure of 2 

 atmospheres acquired a subnormal content in oxygen. These animals then exhib- 



FiG. 258. — Effect of COa on Respiratory Movements of Rabbit. {Scott.) 

 During the first period indicated on the signal line the animal breathed 9.6 per cent. 



CO2 in air, and during the second period 10 per cent. CO2 with 33 per cent, oxygen. 



Time tracing = 2 seconds. 



ited inflammatory changes of the lining cells of the alveoli, similar to those ob- 

 served in pneumonia. A longer exposure to this gas proved fatal in a few days. 

 Facts such as these unmistakably prove that the administration of oxygen is not 

 at all without danger. 



A slight increase in the partial pressure of the carbon dioxid (to 5 per cent.) is 

 reacted against as a rule by an increase in the respiratory rate and amplitude 

 (hyperpnea), but the intensity of the oxidations is not materially changed. ^ In 

 those cases in which a greater output of carbon dioxid has actually been observed, 

 the change seems to be due to the greater activity of the muscles of respiration. If 

 the carbon dioxid in the inspired air is increased to 8 or 10 per cent., dyspnea results; 

 the output of carbon dioxid is diminished and later on also the intake of oxygen. 

 A further increase in the partial pressure of this gas to 15 per cent, leads to an 

 augmentation of these conditions until, at concentrations of from 30 to 40 per cent., 

 a respiratory depression sets in which is soon followed by the death of the animal. 

 At first, therefore, the tendency is to increase the ventilation in the alveoli by 

 hyper efforts at respiration so as to maintain the tension of this gas in the blood. 

 This change is accompanied by a rise in blood pressure which is caused in part by 

 a greater cardiac output and in part by a constriction of the blood-vessels. Later 

 on, however, as the tension of the carbon dioxid is increased to 15 per cent., the 



1 Jour, of Physiol., xxii, 1898, 307. 



2 Speck, Menschl. Atmung., Leipzig, 1892. 



