RESPIRATION. 571 



happen to be. When swallowing has been accomplished the inhibitory influ- 

 ence is removed and respiration is resumed. 



The inhalation of irritating gases may cause respiratory arrest by exciting 

 either the sensory fibres of the trigeminal nerves in the nose or the pneumo- 

 gastric fibres in the larynx and lungs. Some gases affect the former, some 

 the latter, others both. In the rabbit, for example, the introduction of tobacco- 

 smoke into the lungs through a tracheal opening produces no effect upon the 

 respirations, but if injected into the nose respiration is at once arrested. When 

 ammonia is similarly introduced into the lungs the respirations may be either 

 accelerated or diminished, and may be arrested in the inspiratory or the expi- 

 ratory phase, but when drawn into the nose expiratory arrest follows. Some 

 irritating gases arrest respiration in the inspiratory phase, others in the expi- 

 ratory phase. Odorous gases which are powerful and disagreeable may simi- 

 larly cause arrest by acting upon the olfactory nerves. Excitation of the 

 splanchnic nerves causes expiratory arrest ; stimulation of the sciatic and sen- 

 sory nerves in general usually increases the number of respirations, yet under 

 certain circumstances it may cause a decrease and final arrest during expi- 

 ration. 



Stimulation of the cutaneous nerves, as by a cold douche, slapping, etc., 

 causes primarily a tendency to an increase in the number and depth of the res- 

 pirations, but finally causes cessation in the expiratory phase. It is stated that 

 excitation of these nerves is more effective in causing respiratory movements 

 than irritation of the vagi. The influence of external heat is very powerful, 

 and is perhaps the most potent means, under ordinary circumstances, of exciting 

 the respiratory centre. ' The respiratory movements caused by cutaneous irrita- 

 tion, are, however, of the character of reflex spasms rather than of normal 

 movements, and when the excitation is sufficiently strong the movements may 

 be distinctly convulsive. 



Finally, afferent (intercentral) fibres connect the brain-cortex, and probably 

 the ganglia at the base of the brain, with the respiratory centres. 



The Efferent Respiratory Nerves. During ordinary respiration the only 

 efferent or motor nerves necessarily involved are the phrenics, and certain other 

 of the spinal nerves, and the pneumogastrics. Section of one phrenic nerve causes 

 paralysis of the corresponding side of the diaphragm ; section of both phrenics 

 is followed by paralysis of the entire diaphragm. So important are these 

 nerves in respiration that in most cases after section death occurs from asphyxia 

 within several hours. In such cases not only is the work of inspiration thrown 

 upon the other inspiratory muscles, but the effectiveness of the latter is greatly 

 compromised by the relaxed condition of the diaphragm, which permits of its 

 being drawn into the thoracic cavity with each inspiration, thus hindering the 

 expansion of the lungs. If section be made of the spinal cord just below the 

 exit of the fifth cervical nerve, costal movements cease, but diaphragmatic con- 

 tractions continue. The level of the section is just below the origin of the roots 

 of the phrenics, so that the motor fibres for the diaphragm are left intact, but 

 the motor impulses which would have gone out to other inspiratory muscles 



