76 ELEMENTS OF HISTOLOGY. [Chap. x. 



birds (not in man). They are thin, transversely striped, 

 muscular fibres, the central part of which is a con- 

 tinuous mass of protoplasm, with nuclei at regular 

 intervals, the same as is the case with some skeletal 

 muscular fibres of insects. 



97. The muscular fibres forming the proper wall 

 of the heart, the structure of which has been described 

 in the previous chapter, are grouped in bundles 

 separated by vascular fibrous connective tissue. In the 

 ventricles the bundles are aggregated into more or less 

 distinct lamellae. 



Like other striped muscular fibres, those of the 

 wall of the heart are richly supplied with blood-vessels 

 and lymphatics. The endocardium and valves and the 

 pericardium possess their own systems of capillaries. 



The lymphatics form a pericardial and an endo- 

 cardial network, the muscular substance of the heart 

 having numerous lymphatics in the shape of lymph 

 clefts between the muscular bundles, and also typical 

 networks of tubular lymphatics. 



98. The nerve branches of the plexus cardiacus 

 form rich plexuses. In connection with some of 

 them are found numerous collections of ganglion cells 

 or ganglia. These are very numerous in the nerve 

 plexus of the auricular septum of the frog's heart (Lud- 

 wig, Bidder), and in the auriculo-ventricular septum 

 of the frog (Dogiel). In man and mammals numerous 

 ganglia are found on the subpericardial nerve branches, 

 chiefly at the point of junction of the large veins with 

 the heart, and at the boundary between the auricles 

 and the ventricles. 



99. (B) The arteries (Fig. 45) consist of : (a) an 

 endothelial layer lining the lumen of the vessel ; (6) an 

 intima consisting of elastic tissue; (c) a media, con- 

 taining a large proportion of non-striped muscular cells 

 arranged chiefly in a transverse, i.e., circular, manner; 

 and (d) an adventitia composed chiefly of fibrous 



