420 THE HUMAN BODY. 



ments. These nerves give branches to the larynx, the wind- 

 pipe, and the lungs, and might therefore be suspected to 

 have something to do with breathing. Indeed at one time it 

 was maintained that the breathing movements were purely 

 reflex, the afferent fibres running in the pneumogastrics 

 from the lungs to the respiratory centre. That the vagi are 

 not concerned in influencing the respiratory muscles directly 

 is shown by the fact that all of these muscles (except certain 

 small ones in the larynx) contract as usual in breathing after 

 both pneumogastric nerves have been divided. Still, the 

 section of both nerve's has a considerable influence on the 

 respiratory movements; they become slower and deeper. 

 We may understand this by supposing that the resistance to 

 the discharges of the respiratory centre is liable to variation. 

 It may be increased, and then the discharges will be fewer 

 and larger; or diminished, and then they will be more fre- 

 quent but each one less powerful. If the spring, in the 

 illustration used in the preceding paragraph, be made stronger, 

 while the inflow of water to the tube remains the same, the 

 outflows will be less frequent but each one greater; and vice 

 versa. The effect of section of the pneumogastric trunk 

 may, therefore, be explained if we suppose that, normally, it 

 carries up, from its lung branches, nervous impulses which 

 diminish the resistance to the discharges of the respiratory 

 centre ; when the nerves are cut these helping impulses are 

 lost to the centre, and its impulses must gather more head 

 before they break out, but will be greater when they do. 

 This view is confirmed by the fact that stimulation of the 

 central ends of the divided pneumogastrics, if weak, brings 

 back the respirations to their normal rate and force; if 

 stronger makes them more rapid and shallower; and when 

 stronger still, abolishes the respiratory rhythm altogether, 

 with the inspiratory muscles in a steady state of feeble con- 

 traction. That is to say, the resistance to the discharges of 

 the centre being entirely taken away (which is equivalent to 

 the total removal of the spring in our example), the centre 

 sends out uninterrupted and non -rhythmic stimuli to the 

 inspiratory muscles. 



The pneumogastric nerve gives two branches to the 

 larynx; known respectively as the superior and inferior (re- 

 current) laryngeal nerves; the action of these on the respira- 

 tory centre is opposite to that of the fibres from the lungs 



