290 CIRCULATION OF THE BLOOD. 



below by the crescentic upper border of the first rib, behind 

 and to the outside by the scalenus anticus, and to the inside by 

 the trachea and (on the right side) by the oesophagus. In the 

 depth of the hollow, to the outside, lies the subclavian artery 

 on its way to cross outwards over the first rib : the vertebral 

 artery springs from it just as it is about to leave the hollow 

 space. This vessel is the guide to the ganglion which lies on 

 its inner side concealed in a good deal of cellular tissue. To 

 lind it, the most certain method is to seek for the trunk of the 

 sympathetic in the upper part of the space where it lies con- 

 cealed behind the carotid artery, and then to trace it down to 

 the ganglion. All this having been accomplished without 

 bleeding, there is no difficulty in passing a ligature round the 

 ganglion, so that at any desired moment it may be extirpated. 

 The same operation is then performed on the opposite side of 

 the body. Both ganglia having been thus prepared with as 

 little loss of time as possible, the sympathetic and vagus are 

 divided (so as completely to sever the nervous connection be- 

 tween the heart and the central nervous system), and one of 

 the carotids is connected with the kymograph. 



The medulla oblongata is then divided, and comparative 

 observations are made, in the manner already directed, as to 

 the effect of excitation of the peripheral end of the spinal 

 cord on the arterial pressure, and on the frequency of the pulse 

 before and after extirpation of both ganglia. In the one case, 

 the rise of pressure is attended with acceleration ; in the other, 

 the frequency of the contractions of the heart remains un- 

 altered. This result proves, first, that the accelerative influ- 

 ence of the cordon the heart is conveyed by nerves which pass 

 through the ganglia; and secondly, that these nerves are not 

 in constant action. Although the cord, when excited, acts 

 throughout by means of them, their destruction produces no 

 elt'eet on the heart when the cord is quiescent. To complete 

 the proof that the nerves which pass to the heart from the 

 sympathetic trunk, and particularly those which spring from 

 the ganglion, are concerned in shortening the diastolic inter- 

 vals, direct observations are necessary. Such observations 

 were first made by the brothers Cyon, who found that both in 

 the dog and rabbit most of the accelerator fibres reach the 

 ganglion by the nerve which accompanies the vertebral artery. 

 In both animals, but especially in the dog, as has been already 

 stated, the path followed by these fibres from the ganglion to 

 the heart varies considerably in different individuals. The 

 experiments by which these facts have been established are 

 among the most difficult in physiology, and consequently the 

 description of them lies beyond the scope of this work. 



From the preceding experiments and observations, we learn 

 that it is the function of the accelerator nerves to shorten the 



