RESPIRATION 431 



time when there are reasons for believing that the afferent paths are still 

 incapable of conduction, the initial rate of discharge was practically constant, 

 about four per minute in the cat. The same result was observed in some 

 instances in cats when in addition to producing anemia the vagi as well 

 as the region posterior to the corpora quadrigemina were divided (Stewart) . 



It may therefore be assumed that the respiratory center possesses an in- 

 dependent automatic rhythm which is, however, much slower than that 

 characteristic of it when all afferent paths leading to it are intact. 



Accepting the statement that the fundamental rhythm of the inspiratory 

 center is automatic that is, due to a stimulus generated within itself 

 the question at once arises as to the nature of the stimulating agent. By 

 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. 



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 pulmonic 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. 195.) 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 



