264 THE HUMAN BODY. 



when stimulated quicken or strengthen the heart beat. The 

 afferent fibres will be more conveniently studied in connection 

 with nerves of the blood-vessels (Chap. XVIII). 



The Cardio-inhibitory Fibres. These, though running 

 in the neck in what seems to be the main pneumogastric 

 trunk, do not leave the skull in that nerve, but in the spinal 

 accessory (XI cranial nerve), which, it will be remembered, 

 arises in part from the brain and in part from the upper por- 

 tion of the spinal cord. That nerve gives off near the brain 

 a small branch which joins the pneumogastric and runs on in 

 it to near the heart. The fibres may be tracked in the pneu- 

 mogastric by their small size, but more satisfactorily by the 

 Wallerian method. It is then found 1, when the main 

 pneumogastric trunk is divided in the neck all the medullated 

 fibres in it distal to the place of section degenerate; 2, if only 

 the branch joining the spinal accessory to the pneumogastnc 

 be cut, then only some fibres in the pneumogastric stem do- 

 generate, and these fibres are the small medullated set; 3, if 

 the pneumogastric alone be divided above the point where the 

 branch from the spinal accessory joins it, then the large 

 medullated fibres of the cardiac branches of the vagus degen- 

 erate, but the small do not. Hence we conclude that the 

 small fibres come through the accessory. Physiological ex- 

 periment confirms this. Immediately after cutting the main 

 pneumogastric trunk stimulation of its peripheral end checks^ 

 the beat of the heart; but if the stimulation be applied 

 after several days, it has no effect on the heart. If instead 

 of cutting the whole pneumogastric stem we divide only 

 the branch going to it from the accessory, we find similar 

 results: after two or three days (i.e., when the microscope 

 reveals degeneration of the small medullated fibres in the 

 main stem, all the rest being in their normal condition) stim- 

 ulation of it is as absolutely without direct effect on the heart 

 as after complete degeneration of the whole nerve-trunk. In 

 the frog there is no separate spinal accessory nerve; the cardio- 

 inhibitory fibres pass from the brain directly into the pneumo- 

 gastric; but in both frog and mammal their centre lies in a 

 group of nerve-cells of the medulla oblongata known as the 

 cardio-inliibitory centre. 



The cardiac nerve of the frog consists (Fig. 99) of a pneu- 

 mogastric and a sympathetic portion: if it be stimulated the 

 usual result is that the heart is slowed when the stimulus is 



