THE HEART. 



101 



leucocytes, as insisted on by Ehrlich and his supporters, based on the 

 assumption that the leucocytes originate exclusively within the bone- 

 marrow, is open to challenge. As shown by Ebner, all the typical forms 

 of white cells, including the polymorphonuclear leucocytes, appear before 

 the differentiation of the earliest bone-marrow. It is not improbable, there- 

 fore, that all forms of the white cells are related genetically and traceable to 

 common ancestors. 



THE HEART. 



In principle, the heart is a modified blood-vessel, formed by the fusion 

 of two heart-tubes and converted into an efficient organ for propulsion by the 

 unusual development of muscular tissue within its walls. As are the walls 

 of the larger arteries, so also is that of the heart composed of three general 

 layers. The inner of these, the endocardium, consists of an endothelial lining 

 and fibro-elastic tissue. The middle layer, the myocardium, contributes by 

 far the greatest bulk of heart-tissue and is made up of intricately arranged 



Heart muscle 



Blood-vessel 



FIG. 138. Section of endocardium. X 325. 



sheets of cardiac muscle and fibro-elastic tissue. The outer layer, the epicar- 

 dium, the visceral layer of the pericardium, is a stratum of fibro-elastic tissue, 

 covered externally, except at the base where the great vessels join the heart, 

 with endothelium. 



The Endocardium. The endocardium follows all the irregularities 

 of the interior of the heart, lining every recess and covering the free surfaces 

 of the valves, tendinous cords and papillary muscles. It consists of a single 

 layer of endothelial plates and the underlying connective tissue. The latter 

 contains many scattered strands of fibrous tissue and is rich in elastic fibres. 

 The elastic tissue occupies particularly the deeper parts of the endocardium, 

 being almost wanting beneath the endothelium, and in the auricles or atria, 

 where it is most abundant, may be condensed into fenestrated membranes. 

 The deepest layer of the endocardium blends with the connective tissue of 

 the subjacent myocardium. 



The valves of the heart are essentially duplicatures of the endocardium, 

 strengthened in their thicker parts by fibro-elastic tissue. In the case of the 

 atrio-ventricutar valves, this tissue is continuous with the dense fibrous rings 

 Cannuli fibrosi) to which the leaflets are attached. Towards the free margin 

 of the valve, the layers are blended and reduced to a thin fibrous stratum 

 covered on both sides by endothelium. Strands of nonstriated muscle occur 



