4 i8 TEXT-BOOK OF PHYSIOLOGY. 



some investigators it has been assumed that the stimulus is connected with 

 the content or pressure of oxygen, by others with the content or pressure of 

 carbon dioxid, and that the variations in the respiratory rhythm are depend- 

 ent on variations in the pressure of one or the other of these two gases. 

 As a result of a long series of experiments made on animals and human 

 beings, with the respiratory nerve mechanism intact, it is now the generally 

 accepted opinion that the more efficient cause for the respiratory rhythm 

 is an increase in the pressure of carbon dioxid in the blood and hence in the 

 center itself rather than a decrease in the pressure of the oxygen. Whether 

 the pressure of the carbon dioxid be the efficient cause or not of the funda- 

 mental respiratory rhythm, there is abundant evidence that the activity or the 

 irritability of the center is modified to an extraordinary extent by variations 

 in the pressure of the carbon dioxid when the nerve system is intact. Proofs 

 in support of this statement will be given in a subsequent paragraph. 



The first inspiration after birth is supposed to be due to the direct 

 stimulation of the respiratory center by the increase in the carbon dioxid 

 present in the blood, though it may be aided by the cooling of the skin due 

 to vaporization of the amniotic fluid. 



Reflex Stimulation of the Inspiratory Center. Whether the inspira- 

 tory center is automatic in character or not, it may be influenced directly 

 by nerve impulses descending from the brain in consequence of volitional 

 acts or emotional states, and indirectly by nerve impulses brought to it 

 from the general periphery through various afferent nerves, in consequence of 

 agencies acting on their peripheral terminations : e.g., cold applied to the skin, 

 irritating gases to the nasal and bronchial mucous membrane, distention and 

 collapse of the pulmonary alveoli. 



Of all afferent nerves, the vagus appears to be the most influential in 

 maintaining the normal rhythmic discharge of nerve impulses from the 

 inspiratory center, as shown by the effects that follow their separation from 

 the center. (Fig. 199.) Thus, if while the animal is breathing regularly 

 and quietly both vagi are cut, the respiratory movements become much slower, 

 falling perhaps to one-third their original number per minute. At the same 

 time the inspirations become deeper and somewhat spasmodic in character. 

 The duration of the inspiratory movement is also increased beyond that 

 of the expiratory movement. If now the central end of the divided vagus 

 be stimulated with weak faradic currents, the respiratory movements are 

 again increased in frequency and their depth diminished until the normal 

 rate is restored. With the cessation of the stimulation the former condition 

 at once returns. This would indicate that in the physiologic state afferent 

 impulses are ascending the vagus fibers which influence the extent of dis- 

 charge from the inspiratory center, or, in other words, inhibit the inspiratory 

 discharge and lead to an expiratory movement sooner than would other- 

 wise be the case. If, however, the stimulation is increased in strength, 

 the inspiratory movement gradually so exceeds the expiratory that the mus- 

 cles pass into the tetanic state and the chest-walls come to rest in the con- 

 dition of forced inspiration. The vagus apparently contains fibers which 

 are capable of so exciting or augmenting the activity of the inspiratory center, 

 and therefore the extent of the inspiratory movement, as to lead to the con- 

 dition of tetanus of the inspiratory muscles. If, on the other hand, the 



