THE VASCULAR MECHANISM. 207 



nervous system, and that probably in various ways. That palpitation of the 

 heart which is so conspicuous an effect of certain emotions is probably due 

 to the sudden positive action of augmenting impulses, though it may possibly 

 be due, in part at least, to sudden withdrawal of normal, continuous, tonic, 

 and inhibitory impulses. 



In the mammal, then, as in the frog, the heart is governed by two sets 

 of nerves, the one antagonistic to the other. In the dog the roots of the 

 spinal accessory nerve, by which inhibitory fibres leave the central nervous 

 system, consists entirely of inedullated fibres. Among these are fibres of 

 fine calibre, 2/J.-&/J. in diameter, which may be traced down the trunk of 

 the vagus, along the branches going to the heart, right down to the heart 

 itself. There can be little doubt that these medullated fibres of fine calibre 

 are the inhibitory fibres of the vagus, and indeed there is evidence which 

 renders it probable that the inhibitory fibres of the heart are always medul- 

 lated fibres of fine calibre, which continue as medullated fibres right dow ? n 

 to the heart, but eventually lose their medulla in the heart itself. 



The anterior roots of the second and third dorsal nerves, and the (white) 

 rami communicantes belonging to them, which, as we have just seen, con- 

 tain in the dog augmentor fibres, also consist exclusively of medullated 

 fibres. But the nerves which convey the augmenting impulses from the 

 lower cervical ganglion, or from the annulus of Vieussens to the heart, 

 consist of non-medullated fibres. Hence, the augmentor fibres must have 

 lost their medulla, and become continuous with non-medullated fibres some- 

 where in their course along the sympathetic chain. It is probable that the 

 change occurs in the ganglion stellatum and lower cervical ganglion, and 

 it is further probable that the change is effected by the medullated fibre 

 passing into one of the ganglion cells, and so losing its medulla, the im- 

 pulses which it conveys passing out of the nerve cell by one or more of the 

 other processes of the cell which are continued on as non-medullated fibres. 

 Cf. 94. 



In the dog then these two sets of nerve fibres, antagonistic to each other 

 in function, differ in structure, the augmentor fibres early losing their 

 medulla, and hence being over a large part of their course non-medullated 

 fibres, whereas the inhibitory fibres are medullated fibres which, though they 

 may pass by or through ganglia (as the ganglion jugulare and ganglion 

 trunci vagi), do not lose their medulla in these ganglia, but remain as 

 medullated fibres right down to the heart. And this difference in structure 

 appears to hold good for all mammals, and is possibly true for vertebrates 

 generally. 



147. The question, What is the exact nature of the change brought 

 about by the inhibitory and augmenting impulses respectively on their 

 arrival at the heart? or, in other words, by virtue of what events produced 

 in the heart itself do the impulses of one kind bring about inhibition, 

 of the other kind augmentation ? is a very difficult one, which we can- 

 not attempt to discuss fully here. We may, if we please, speak of an 

 " inhibitory mechanism " placed in the heart itself, but we have no exact 

 knowledge of the nature of such a mechanism. Still less do we possess 

 any satisfactory information as to an augmenting mechanism. It has been 

 suggested that some of the ganglia in the heart serve as such an inhibi- 

 tory (or augmenting) mechanism ; but there is evidence that the inhibitory 

 impulses produce their effect by acting directly on the muscular fibres, or at 

 all events do not produce their effect by acting exclusively on any gan- 

 glia. One evidence of this kind is supplied by the action of the drug 

 atropine. 



If, either in a frog O r a mammal, or other animal, after the vagus fibres 



