RESPIRA TION 



633 



with the respiratory centre by means of the vagus nerves. They have been 

 referred to above (page 387) in the discussion of the meaning of the electrical changes 

 in the peripheral ends of the vagus nerves, as photographed by Einthoven (see 

 Fig. 106, p. 386). The relative parts played by the nervous and the chemical 

 factors were the subject of investigations by F. H. Scott (1908). Although 

 expiration, under normal conditions, is almost entirely a passive movement, due to 

 return of the mechanism to its equilibrium position, it is well known that, in 

 dyspnoea, co-ordinated action of muscles antagonistic to those of inspiration takes 

 place in the expiratory phase. We must, therefore, suppose that the respiratory 

 centre is double, inspiratory and expiratory, and each of these will be capable 

 of excitation and of inhibition, forming a system subject to double reciprocal 

 innervation of the kind described above (page 498) with reference to the flexors and 

 extensors of the limbs. 



Although it had been known for a long /I /] / 



time that respiratory movements could be in- III 



fluenced reflexly by stimulation of various afferent 

 nerves, and especially of the vagus, the first 

 clear and systematic results were obtained by 

 Head (1889), using a slip of the diaphragm, 

 which is so attached in the rabbit as to allow 



Curve IK 



FIG. 195. EFFECT OF SHORT SUCTION. As above in 

 regard to meaning of tracings. 



Note that deflation excites inspiratory movements. 



(Head, 1889, PI. 2, Fig. 9.; 



FIG. 196. DIAGRAM OF EFFECTS 



OF VENTILATION ON RESPIRA- 

 TORY REFLEXES. Inspira- 

 tion upwards. 



A, Normal tracing of diaphragm slip. 



B, Inflation. Standstill in relaxation. 



C, Suction. Standstill in tonic con- 



traction. 



D, Combination of the two. Cessa- 



tion of automatic movements. 

 Diaphragm in mean state of tonus. 



(Head, 1889, p. 31.) 



a tracing lever to be connected without interference of the nervous supply to it or 

 disturbance of the normal respiratory movements. The original paper must be 

 consulted for numerous points to which we cannot refer here. The main question 

 is as to what happens when the vagus endings in the lungs are stimulated by 

 expansion and collapse of these organs. Hering and Breuer (1868) had already 

 made experiments of this kind with a less perfect method, and founded a theory 

 of self-regulation by the vagus nerves. Head showed definitely that the effect 

 of increase in the volume of the lungs is to inhibit all inspiratory movements, and 

 that decrease of volume has the opposite effect of exciting inspiration (see Figs. 

 194 and 195). Of course, as we should expect from what is known now of the 

 part played by carbon dioxide, either inflation or suction, repeated periodically, 

 produces stoppage of spontaneous respiration, owing to removal of carbon dioxide, 

 but, with the vagi intact, the stoppage is in the position of inspiration ( = excitation 

 of inspiratory centre), with suction, and the opposite with inflation (see Head's 

 diagram, Fig. 196). 



