RESPIRATION. 567 



Afferent impulses exercise an important, and practically a continuous, influ- 

 ence. After section of one pneumogastric nerve the respirations are somewhat 

 less frequent ; after section of both nerves the respirations become considerably 

 less frequent and deeper and otherwise changed. If we stimulate the central 

 end of one of these cut nerves below the origin of the laryngeal branches by 

 a current of electricity of moderate intensity, the respiratory rate may be in- 

 creased, and we may be able to restore, or even exceed, the normal frequency. 

 The fact that section of these nerves is followed by a diminution of the rate 

 and that excitation of the central end of the cut nerve causes an increase leads 

 us to believe that the pneumogastric nerves are continually conveying impulses 

 from the lungs to the respiratory centre, which impulses in some way increase 

 the number of discharges, and thus the respiratory rate. The centre may be 

 excited or depressed by excitation of the cutaneous nerves and the sensory 

 nerves in general ; thus, external heat accelerates, while a dash of cold water 

 may either accelerate or inhibit, respiratory movements. Excitation of the 

 glosso-pharyngeal nerves inhibits the respirations. Such inhibition occurs 

 during deglutition to avoid the risk of introducing foreign bodies into the 

 larynx. Similar respiratory inhibition may be induced by excitation of the 

 superior laryngeal nerves, when, if the degree of irritation be sufficiently 

 strong, complete arrest of the respiratory movements may occur. Strong irri- 

 tation of the olfactory nerves and of the fibres of the trigemini distributed to 

 the nasal chambers excites expiration and may be followed by complete inhibi- 

 tion of the respiratory movements ; strong irritation of the optic and auditory 

 nerves excites inspiratory activity ; and irritation of the sciatic nerve causes an 

 increase of the rate, and may or may not affect the depth of breathing. 



The study of the rhythmic activity of the respiratory centre is further 

 complicated by the fact that there is not only a rhythmic sequence of the res- 

 pirations, but a rhythmic alternation of inspiratory and expiratory move- 

 ments. While it is true that in ordinary quiet expiration but little of the 

 muscular element is present, yet forced expiration is a well-defined co-ordinated 

 muscular act. The mechanism whereby this alternation is brought about is 

 not understood. Some believe that the pneumogastric nerves contain both 

 inspiratory and expiratory fibres which are connected with corresponding parts 

 of the respiratory centre and alternately convey their respective impulses to 

 the centre, inspiratory impulses being excited during expiration and expiratory 

 impulses during inspiration (p. 505). These impulses are, however, not indis- 

 pensable to the alternation of inspiration and expiration, because these acts 

 follow each other regularly, even after the isolation of the respiratory centre 

 from the lungs by section of the pneumogastric nerves. 



Thus we may conclude that the rhythmical discharges from the centre are 

 due primarily to an inherent property of periodic activity of the nerve-cells 

 constituting the respiratory centre and maintained by the blood, and that the 

 rhythm, rate, and other characters of these discharges may be affected by the 

 will and the emotions, by the composition, supply, and temperature of the 

 blood, and by various afferent impulses. The chief factors are, under ordi- 



