396 THE HUMAN BODY. 



matter called protoplasm, and perhaps only of some varie- 

 ties of it. 



The Relation of the Pneumogastric Nerves to the Res- 

 piratory Centre. We have next to consider if any pheno- 

 mena presented by the living Body give support to the 

 resistance theory of the respiratory rhythm. A very im- 

 portant collateral prop to it is given by the relation of the 

 pneumogastric nerves to the rate and force of the respira- 

 tory movements. These nerves give branches to the larynx, 

 the windpipe, and the lungs, in addition to numerous other 

 parts, and might therefore be suspected to have something 

 to do with breathing. That they are not concerned in in- 

 fluencing 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 nerves has a considerable influence on the respira- 

 tory movements; they become slower and deeper. We may 

 understand this by supposing that the resistance to the dis- 

 charges 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 frequent 

 but each one less powerful. If the spring, in the illustra- 

 tion 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 respirator} 

 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- 



