CHAP, iv.] THE VASCULAR MECHANISM. 295 



with which are small groups of cells, to the auricular walls, and to 

 the sinus walls, and on the other hand the two main nerves 

 running along the septum, in connection with which are small 

 ganglia which may be called ' septal ' ganglia. From the latter, 

 Bidder's ganglia, fibres unaccompanied except for a short distance 

 by nerve cells, pass to the substance of the ventricle, and possibly 

 to the bulbus arteriosus. 



The fibres forming the vagus nerves as they run along the 

 superior venae cavse are composed of medullated and non-medul- 

 lated fibres, the latter being chiefly if not wholly derived from the 

 sympathetic system. Many of the medullated fibres lose their 

 medulla in Remak's ganglion, for non-medullated fibres are found 

 in much larger proportion in the septal nerves, running to 

 Bidder's ganglia ; the fine fibres which pass from Bidder's ganglia 

 to the substance of the ventricle are exclusively non-medullated 

 fibres. The nerve cells in the sinus ganglia and along the ends of 

 the vagus nerves, as well as some of the cells of the ganglia 

 scattered over the septum, are of the kind previously ( 98) 

 described as spiral cells. The cells composing Bidder's ganglia, 

 as well as many of the cells in the septum, are said to be bipolar 

 and fusiform. 



In the mammal, the nerves going to the heart are derived on 

 the one hand from the vagus and on the other hand from the 

 sympathetic chain. Thus in man the upper, middle and lower 

 cervical ganglia (or the cord between them) give off the upper 

 lower and middle sympathetic cardiac nerves respectively, while 

 the trunk of the vagus gives off cervical cardiac branches in the 

 neck and thoracic branches in the thorax ; the recurrent laryngeal 

 also gives off branches especially on the left side, and there is as 

 well a cardiac branch of the external division of the superior 

 laryngeal. The nerves from these two sources, vagal and sym- 

 pathetic, form near the roots of the aorta and pulmonary artery, 

 the cardiac plexuses, superficial and deep, the two sources 

 mingling largely here and also to a certain extent before the 

 plexuses are reached. From the plexuses fibres are given off to all 

 parts of the heart, vena? cavse, pulmonary veins, auricles and ventri- 

 cles, a large number of the fibres destined for the latter forming 

 the coronary plexuses around the coronary arteries ; some of the 

 fibres pass to the walls of the aorta and pulmonary artery. In 

 other mammals we find the same double supply reaching the 

 heart by means of the cardiac plexuses, the details differing in 

 different animals ; we shall give later on some details concerning 

 the dog, since much of our knowledge of the nervous working 

 of the mammalian heart has been gained by experiments on this 

 animal. Ganglia are abundant on the superior vena cava and are 

 also found on the pulmonary veins, in the walls of the auricles, 

 in the auriculo-ventricular groove and in the basal portion of 

 the ventricles ; further, according to some observers, in contrast to 



