CHAP, iv.] THE VASCULAR MECHANISM. 313 



especially affected, more so than that of the ventricles, and it may 

 sometimes be observed that the auricles are brought to complete 

 quiescence while the ventricles still continue to beat ; the latter 

 now exhibit that independent rhythm of which we spoke in 156. 

 In a somewhat similar manner the stimulation of the vagus, by 

 affecting the rhythm of the auricles more than that of the ventricles, 

 may lead to a want of coordination between the two, the especially 

 slowed auricles beating at one rate, the ventricles at another. 

 It is indeed maintained by some that the vagus acts directly on 

 the auricles only, the changes in the ventricles being of a secondary 

 nature, caused by the changes in the auricles. 



When the output from the ventricles during vagus stimulation 

 is measured, by the cardiometer or otherwise, it is found, as might 

 be expected, that this is lessened. The diminution during a given 

 period may be due to the mere slowing of the beat; but the 

 individual pulse volume is in some cases at least also lessened. 

 It may by the same method be observed that the quantity remain- 

 ing in the ventricle at the end of the systole is increased ; the 

 ventricle appears to expand more during diastole. Of the effects 

 thus produced on the circulation we shall speak later on. 



We may now turn to some further details concerning the 

 course of these inhibitory fibres. They run in the trunk of the 

 vagus ; this is clear not only in the case of an animal like the 

 rabbit, in which the vagus runs separate from the cervical sym- 

 pathetic but also in the case of the dog, in which the two nerves 

 are more or less bound up together. Leaving the vagus by the 

 cardiac branches, they reach the cardiac tissues by the cardiac 

 plexuses. When we trace the fibres in the other direction to- 

 wards the central nervous system, we have to bear in mind that 

 the fibres which compose the trunk of the vagus have, as we shall 

 see in studying the central nervous system, two distinct central 

 origins. On the one hand, there are the fibres which are the 

 proper vagus fibres which, leaving the spinal bulb, pass through 

 both the jugular ganglion and trunk ganglion (Fig. 72 r. GJ. 

 G. Tr. Vg.). On the other hand, there are fibres which, belonging 

 to the spinal accessory nerve (Sp. Ac.) and to what we shall learn 

 to speak of as the bulbar division of that nerve, pass after leaving 

 the spinal bulb to the trunk ganglion of the vagus, and thence 

 form part of the vagus trunk. Now it is these fibres of the spinal 

 accessory nerve and not the proper vagus fibres which supply the 

 inhibitory fibres to the heart. Thus if the bulbar roots of the 

 spinal accessory be divided, those of the vagus proper being left 

 intact, the spinal accessory fibres in the vagus trunk degenerate, 

 and when this has taken place stimulation of the vagus fails to 

 produce the ordinary inhibitory effect. 



Within the spinal bulb" these inhibitory fibres are connected, 

 in the mammal as in the frog, with a cardio-inhibitory centre ; and 

 in the mammal as in the frog inhibition may be brought about 



