CHAP, iv.] THE VASCULAR MECHANISM. 279 



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

 bul bus arteriosus. 



In the mammal, the arrangement appears to conform more 

 closely to the general plan described above. The several cardiac 

 nerves from the sympathetic chain together with branches from 

 the vagus, including fibres from the recurrent laryngeal, form the 

 superficial and deep cardiac plexuses below and beneath the arch 

 of the aorta. From these plexuses fibres are distributed to the 

 superior vena cava and to the pulmonary veins and thence to the 

 various parts of the heart. 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 the frog's heart, a number of small ganglia may be 

 observed over a large part of the ventricle far down towards 

 the apex. The auricular septum, at least in its central parts, 

 is free from ganglia. The nerves and ganglia lie for the most part 

 superficially immediately under the pericardium. 



In the frog, the fibres forming the vagus nerves as they run 

 along the superior venae cavse are composed of medullated and 

 non-medullated fibres, the latter being chiefly if not wholly derived 

 from the splanchnic or sympathetic system. Medullated fibres, 

 with a larger proportion of non-medullated fibres are found in 

 the septal nerves, running to Bidder's ganglia, but the fine fibres 

 which pass from Bidder's ganglia to the substance of the ven- 

 tricle 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 fibres passing to the heart are also 

 medullated and non-medullated. Some of the medullated fibres 

 are of fine calibre, may be traced back to the vagus, and appear to 

 be fibres of which we shall speak presently as inhibitory. Others 

 of the medullated fibres are of larger calibre, and some of these at 

 all events appear to be sensory or at least afferent in function. 

 Of the non-medullated fibres, some may be traced back along the 

 cardiac nerves to the inferior cervical ganglion and are of the kind 

 we shall speak of as augmenting. In contrast to the frog many of 

 the fibres in the ventricle (where they lie close under the peri- 

 cardium), are medullated, and it is probable that these are afferent 

 fibres. 



The cells forming the various ganglia scattered over the 

 mammalian heart may perhaps be classed as unipolar, and 

 multipolar, the former being especially connected with medullated 

 fibres, the one class being prominent in one situation, the other 

 in another. 



