RESPIRATION 42? 



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 

 isjmjnCTea^e 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. 



Reflex Stimulation of the Inspiratory Center. Whather 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. 



The Relation of the Vagus Nerves to the Inspiratory Center. 

 The vagus nerves, of all the afferent nerves, appear 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. 198.) 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 one of the divided 

 vagi be stimulated with weak induced electric 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 seem to indicate that the 

 vagus nerve contains nerve-fibers which, under physiologic conditions, trans- 

 mit nerve impulses which inhibit the inspiratory discharge and lead to an 

 expiratory movement sooner than would otherwise be the case, and thus 

 maintain the normal rate and extent of the inspiratory discharge. 

 stimulus to the development of these nerve impulses is generally believed 

 to be the distension of the air cells at the beginning of the inspiratory 

 movement. 



If however the central end of the divided vagus be stimulated with 

 induced electric currents of moderate intensity, the opposite effect is pro- 

 duced, viz.: an increase in the extent of the inspiratory movement ana 

 decrease in the extent of the expiratory movement until the inspiratory 

 muscles pass into the tetanic state and the chest walls come to rest 

 condition of a forced inspiration. This would indicate that the vagus nen 

 also contains nerve-fibers which, with the proper degree of stimula 

 capable of so exciting or augmenting the activity of the inspiratory ce 

 and therefore the extent of the inspiratory movement, as to lead i 

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



