528 



A X A T< MY OF VERTEBRATES. 



sent off, so that few reach the apex of the column : but the mould- 

 ing of the ventricle about these is not the cause of the conical 

 figure of the heart, since this obtains where no such mammillary 

 columns are present. (Vol. i. figs. 334-340.) 



The heart is lined by a membrane, ' endocardium,' rather 

 thicker and more opaque in the left than in the right cavities, 

 especially in the auricle : thinnest on the muscular projections, 

 both pectinate and columnate. The chief layer consists of a close 

 network of elastic fibres, lined by a stratum of polygonal epithe- 

 lial scales, constituting the free surface ; and attached by an 

 areolar tissue to the muscular coat. This is covered by the 

 reflected serous layer of the heart-bag, called ( ectocardium.' 



The disposition of the intervening muscular fibres has been best 

 illustrated in relation to the human heart. Those of the auricles 



form a superficial layer, 

 fig. 408, common to both 

 cavities, and also a deep 

 layer, fig. 409, proper to 

 each. 



The superficial layer 

 includes the transverse 

 band of fibres, fig. 408, D, 

 expanding as it passes to 

 the right, RA, and left, LA, 

 auricles. The deeper fibres 

 appear at the parts not covered by the superficial ones. Some, H, 

 arising from the ' annultis aorticus,' K, K, arch over the auricle, be- 

 neath D, contributing some fibres to the septum, at s : other arched 

 or ' looped' fibres, r, curve over the auricles and are attached by 

 both extremities to the auriculo-ventricular rings AV and AA : a 

 third series, C, surround the auricular appendages, AA, and encircle 

 the terminations of the superior, cs, and inferior, Ci, venrc cavre. 



The winding or convolute disposition of these so-called ' an- 

 nular fibres' is exemplified in fig. 409. The superficial and 

 deep-seated fibres are, however, continuous, at parts of their 

 course : those marked E, fig. 408, of the former series, wind 

 round the left auricle LA, and are continuous, with some inter- 

 vening attachment to the aortic root K, with the ascending band 

 r : a posterior band is shown at G, fig. 409, passing over the left 

 auricle LA, and along the posterior border of the appendix A : 

 some of the fibres, on reaching the anterior border, quit the 

 band G, to join the fibres d forming the apex : other detachments 

 from the band y, encompass the terminations of the pulmonary 



f\'\ 



Muscular coat of auricles. CLXXXVIII" 



