3?8 TEXT-BOOK OF PHYSIOLOGY. 



dioxid does not account for the increased rate of breathing. Emo- 

 tional states temporarily increase respiratory activity. With their 

 disappearance the normal condition returns. 



Apnea. If the respiratory movements be voluntarily increased 

 in frequency and depth for a short time it will be found on cessation 

 that for a variable length of time the respiratory mechanism remains 

 in a condition of complete rest or inaction. ^^QZpJiiisxQniDlete cessation 

 of activity the term apnea is givertX The same phenomenon is 

 "witnessed in animals when the lungs are rapidly inflated with air by 

 means of bellows. At one time this was attributed to an excess of 

 oxygen in the blood (the result of the forced ventilation of the lungs), 

 complete saturation of the plasma and hemoglobin, in consequence of 

 which the respiratory center remained inactive. This has been dis- 

 proved, however, by modern chemic analyses of the blood. The 

 condition is now attributed: 



1. To increased ventilation of the lungs and an increased percentage 



of oxygen in the alveoli, as a result of which the normal percentage 

 of oxygen in the blood can be maintained for a longer period than 

 usual. 



2. To a stimulation of the peripheral terminations of the pneumo- 



gastric nerve whereby the discharge of nerve impulses from the 

 respiratory center is temporarily inhibited. Division of the 

 pneumogastric nerve prevents the development of the apneic 

 condition. 



Dyspnea . Excessive__ancL laborious ^xespiratorv movements 

 constitute a condition known as (fysjmea. ' Movements of this char- 

 acter indicate thatJjieJalwd^ or overcharged 

 with carbon dioxid. In either case the excitability of the respiratory 

 center is abnormally heightened. These conditions of the blood 

 may be caused : (i) By all those pathologic conditions of the respiratory 

 apparatus which limit the free entrance of oxygen into and the free 

 exit of carbon dioxid from the blood; (2) by those alterations in the 

 composition of the air and subsequently in the blood which arise when 

 the individual is confined in a space of moderate size with imperfect 

 ventilation. The want of oxygen in the blood gives rise to more 

 forcible inspirations; the presence of CO 2 in excess, to more forcible 

 expirations showing that the former condition affects the inspiratory 

 portion of the respiratory center, the latter condition the expiratory 

 portion. A deficiency in the amount or quality of the hemoglobin 

 is usually attended with dyspnea. 



Asphyxia. If the state of the blood observed in dyspnea be ex- 

 aggerated, that is, if the decrease in the .percentage of oxygen and 

 the increase in the percentage of carbon dioxid become more 

 marked, the respiratory movements becom^mjojje_Jaborious. A 

 continuance of this changed composition of the blood eventuates 



