THE RESPIRATORY CENTER. 755 



If one lung is atelectatic (devoid of air), the pulmonary fibers of the vagus 

 of the same side are inirritable. Section of the vagus upon the side of the healthy 

 lung acts, therefore, in the same way as section of both vagi. 



The inhibitory -fibers which act upon the center for the respiratory 

 movements pass in the superior and inferior laryngeal nerves to the res- 

 piratory center. The recurrent fibers are inactive in deep narcosis. 



Even direct electrical, mechanical or chemical irritation of the center itself 

 may inhibit respiration, perhaps because the irritation affects the central ex- 

 tremities of the inhibitory fibers at their point of entrance into the ganglia. 



Irritation of the inhibitory fibers or their central stumps causes, 

 therefore, slowing and even cessation of breathing in expiration. Ir- 

 ritation also of the nasal branches of the trigeminus, and the orbital 

 branches, as well as the olfactory and the glossopharyngeal, causes 

 arrest of breathing in expiration, as does also irritation of the pulmonary 

 fibers of the vagus by the introduction of certain irritant gases into 

 the lungs. Chemical irritation of the trunk of the vagus, by means of 

 weak solutions of sodium carbonate, induces especially expiratory inhibi- 

 tion of breathing; mechanical irritation (rubbing with a glass rod), in- 

 spiratory inhibition. Also the irritation of sensory cutaneous nerves, 

 particularly of the chest and the abdomen, for example by a sudden cold 

 douche, and also of the splanchnic nerve, causes arrest in expiration, the 

 former often after preceding clonic spasm of the respiratory muscles. 

 The influence of irritation of sensory nerves upon respiration is in 

 general widely distributed, thus, for example, that of the sensory fibers 

 of the phrenic nerve, the heart, the aorta, the abdominal viscera. The 

 slowing of the respiration in conjunction with pressure upon the brain 

 is particularly noteworthy, the breathing not rarely becoming labored 

 and stertorous. 



In man irritation of the nasal mucous membrane in inspiration causes at 

 first inhibition of breathing in the phase present at the time ; then follows inspira- 

 tion. During the period of reflex slowing of respiration the amount of work 

 performed by the respiratory muscles is altered, and particularly the work in the 

 slow respirations is increased through fruitless efforts at inspiration. On the 

 other hand, it has been found that the volume of gases interchanged in the lungs 

 remains the same during corresponding periods, and that, also, the respiratory 

 interchange of gases is at first not altered directly. 



Under normal conditions the pulmonary branches of the vagus appear 

 to affect the respiratory centers through a mechanism of self-regulation 

 in such a manner that the inspiratory dilatation of the lungs, and the 

 associated rarefaction of the contained air, exerts a mechanical irri- 

 tation upon the nerve-fibers stimulating the expiratory center reflexly. 

 Conversely, the expiratory diminution in the size of the lungs, and the 

 resulting increase in intrapulmonary air-pressure, causes irritation of the 

 nerve-fibers passing to the inspiratory center. These fibers are situated 

 in the posterior portion of the true trunk of the vagus. 



According to Lewandowsky there is a special expiratory center, although in 

 normal breathing the inspiratory center alone is active, rhythmically stimulating 

 the inspiratory muscles and then permitting them to relax. 



Deglutitional breathing, that is, a slight contraction of the dia- 

 phragm after each act of swallowing, occurs as an irradiation from the 

 irritation of the swallowing-center to the respiratory center. 



The Excitation of the First Respiratory Movements. The fetus im- 

 mediately after birth is in a state of apnea, as oxygen is abundantly 



