250 THE HUMAN BODY. 



Under certain conditions the isolated apex of the frog's 

 heart gives rhythmic automatic beats; cardiac muscle lias 

 retained more automaticity than ordinary muscle, though 

 not so much as nerve-cells. In any case the cause of the 

 heart's beat lies in the heart itself, though controlled as to 

 rate and force by nerves from elsewhere. 



Nerves Slowing the Heart's Beat. Each pneumogas- 

 tric trunk sends several branches to the heart. Certain of 

 these contain fibres which when excited slow, or even alto- 

 gether stop, the beat of the heart and are hence known as 

 the cardio-inliibitory fibres. 



If one pneumogastric trunk be divided as it runs down 

 the neck and its peripheral, or lower, end be stimulated 

 feebly the heart's beat becomes less frequent, while a more 

 powerful stimulation will completely stop it for a few 

 seconds, as if its muscles were suddenly paralyzed. If the 

 experiment be performed upon a narcotized animal, the heart 

 of which is at the same time exposed by opening the chest, 

 it will be seen that during the stoppage the heart lies flabby 

 and relaxed in diastole: the excitation of the nerve does 

 not stop the heart's beat, as might perhaps be supposed, by 

 keeping it in a state of permanent tetanic contraction, but 

 it annuls its contractions and throws it into a state of rest ; 

 the nerve-fibres concerned are not excitant but inhibitory, 

 stopping instead of calling forth the activity of the part on 

 "which they act. Whether their influence is exerted di- 

 rectly on the muscular fibres of the heart or upon it 3 in- 

 trinsic ganglia, abolishing their automatic activity and so 

 cutting off the stimuli which normally radiate from them 

 to the muscles, is not certainly known, but the former view 

 is probably the correct one. The pneumogastric fibres 

 seem to govern the nutrition of the heart muscle; the 

 stoppage produced by their stimulation is nearly always 

 temporary and followed by more rapid and powerful 

 beats. 



These cardio-inhibitory fibres originate in a collection of 

 nerve-cells in the medulla oblongata known as the cardio- 

 inhibitory centra This centre is automatic and always in 

 a state of slight excitation, feebly stimulating the fibres 

 proceeding from it and slightly slowing the heart's beat. 



