296 THE NERVES OF THE HEART. [BOOK i. 



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 said to be free 

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

 superficial immediately under the pericardium. 



The fibres passing to the heart are, as in the frog, both 

 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 sympathetic system ; of these some again 

 are of the kind we shall speak of as augmenting. Though, as in 

 the frog, the proportion of non-medullated to medullated fibres 

 increases peripherally, yet in contrast to the frog many of the 

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

 cardium), are medullated; 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. 



The Development of the Normal Beat. 



154. The heart of a mammal or of a warm blooded animal 

 generally ceases to beat within a few minutes after being removed 

 from the body in the ordinary way, the hearts of newly-born 

 animals continuing however to beat for a longer time than those 

 of adults. Hence, though by special precautions and by means of 

 an artificial circulation of blood, an isolated mammalian heart may 

 be preserved in a pulsating condition for a much longer time, our 

 knowledge of the exact nature and of the causes of the cardiac 

 beat is as yet very largely based on the study of the hearts of 

 cold blooded animals, which will continue to beat for hours, or 

 under favourable circumstances even for days, after they have 

 been removed from the body with only ordinary care. We have 

 reason to think that the mechanism by which the beat is carried 

 on varies in some of its secondary features in different kinds of 

 animals : that the hearts, for instance, of the eel, the snake, the 

 tortoise and the frog, differ in some minor details of behaviour, 

 both from each other and from those of the bird and of the mammal ; 

 but we may, at first at all events, take the heart of the frog as 

 illustrating the main and important truths concerning the causes 

 and mechanism of the beat. 



