THE NERVOUS REGULATION OF RESPIRATION 533 



757 per cent. Furthermore, it is a matter of common experience that 

 the breath can be held for only a brief period of time, obviously be- 

 cause- the tension of the carbon dioxid in the blood gradually attains 

 so great a stimulating power upon the respiratory center that it can 

 no longer be subdued by volitional efforts. A longer respiratory stand- 

 still may be effected either by taking several deep breaths beforehand 

 or by inhaling pure oxygen. These procedures are intended to remove 

 much of the superfluous carbon dioxid from the lungs and to supply 

 them with enough oxygen to postpone the excitatory influence of the 

 waste products. It is evident, therefore, that the respiratory center 

 is under the direct influence of the blood traversing it. As long as the 

 carbon dioxid tension of the latter remains normal, the respiratory 

 movements retain their eupneic character, while any increase in the 

 tension of this gas is immediately followed by hyperpneic and dyspneic 

 breathing. The tendency is to adjust the depth and frequency of the 

 respiratory movements in such a way that the pulmonary ventilation 

 is always kept the same. Any changes in the gas content of the 

 blood, whether brought about by internal or external causes, affect 

 the center directly and are immediately compensated for by increasing 

 or decreasing its automatic activity. 



The reflex regulation of respiration is made possible by a multitude 

 of afferent impulses, which take their origin in different receptors. 

 Thus, it is a matter of common experience that the amplitude and 

 frequency of the respiratory motions may be varied not only by sudden 

 changes in the intensity of the light and unusual auditory impacts, 

 but also by sensations of smell, taste, touch, pain and temperature. 

 In addition, the automaticity of the respiratory center may be altered 

 by impulses conveyed to it from the psychic centers of the cerebrum. 

 The latter, therefore, must be classified in large part as volitional dis- 

 charges which reach this center by way of diverse transcortical paths. 

 To this class also belong the impulses arising in consequence of emo- 

 tional conditions. 



A cold bath most generally produces a deepening and acceleration 

 of the respiratory movements, while the inhalation of irritating emina- 

 tions leads to an almost instantaneous respiratory standstill. Very 

 similar modifications follow the excitation of the receptors situated in 

 the realm of the splanchnic and sexual organs, but it would lead us 

 altogether too far to discuss these reactions in detail, and besides, their 

 analysis most generally presents no serious difficulty. A certain num- 

 ber of them, however, merit special consideration, because they origi- 

 nate along the pulmonary passage and influence respiration in a most 

 decisive manner. Reference is now had particularly to the acts of 

 sneezing and coughing, resulting in consequence of the excitation of 

 the lining membrane of the nasal, pharyngeal and laryngeal cavities. 

 In accordance with the innervation of these parts, it may be surmised 

 that these reflexes are effected principally with the help of the vagi 

 nerves, which contain afferent as well as efferent respiratory fibers. 



