VESSELS OF THE HEART. 



703 



Muscular tissue 



Fibro- 

 elastic 

 tissue of 

 valve- 

 leaflef 



Endo- 

 thelium 



of the elastica into two strata, a thin subendotheHal layer practically free from elastic 

 fibres and a broad layer in which the elastica predominates. The deepest stratum 

 of the endocardium is continuous with the endomysium that penetrates between the 

 fibres of the heart muscle. 



The valves consist of duplicatures of the endocardium, in their thicker parts 

 strengthened by an intermediate middle layer of fibro-elastic tissue prolonged from 

 the fibrous rings of attachment. Towards the free margins of the valves all three 

 layers are blended and reduced to a thin fibrous stratum covered by endothelium. 

 In the auriculo-ventricular leaflets, the fibro-elastic tissue of the chordae tendineae is 

 continuous with the middle layer, while meagre peripheral bundles of muscle may be 

 present beneath the .endocardium. Al- 

 though thinner than the auriculo- Fig. 667. 

 ventricular, the pulmonary and aortic 

 semilunar valves possess essentially the 

 same structure, the endocardial layer, 

 however, being thickened to produce 

 the noduli Aurantii. 



In addition to the structural de- 

 tails of the fibres composing the myo- 

 cardium already described in connec- 

 tion with the general histology of 

 muscle (page 462), it may be noted 

 that in the immediate vicinity of their 

 nuclei the fibres of the heart- muscle 

 constantly contain accumulations of 

 undifferentiated sarcoplasm in which 

 lie groups of highly refracting brown- 

 ish granules that, under moderate 

 magnification, appear as pigment at 

 the poles of the nuclei. The muscle- 

 fibres, branching into net-works with 

 long narrow meshes, are held together 

 by delicate lamellae of connective tis- 

 sue, the endomysium, which, together 

 with the more robust septa that as the 

 perimysium invest the muscular bun- 

 dles, support the blood-vessels. The 

 relation of the capillaries to the muscle- 

 substance is unusually intimate, the 

 capillary loops often modelling the sur- 

 face of the fibres, lying in deep grooves 

 almost completely enclosed by the sur- 

 rounding sarcous tissue (Meigs). Al- 

 though much less constant and typical 

 than in the ventricular myocardium of 

 many of the lower animals, as the 

 sheep, goat, and ox, the imperfectly 

 differentiated fibres, known as fibres 

 of Purkinje, are represented in the 

 human heart-muscle by subendocardial fibres. There is reason to believe that these 

 fibres are related to the co-ordinating auriculo-ventricular bundle of His (page 701.) 



The Blood-Vessels and Lymphatics of the Heart.— The heart receives 

 its blood-supply through the two coronary arteries which arise from the systemic 

 aorta immediately above its origin, the return flow being by the coronary veins which 

 open into the right auricle by the coronary sinus. Both these sets of vessels will be 

 described later (page 728), but it may be pointed out here that the branches of the 

 coronary artery upon the surface of the heart are, as a rule, all end-arteries, — that is, 

 arteries which form no direct anastomoses with their neighbors. Practically no 

 blood can be carried directly, therefore, by the left coronary artery into the territory 



Chorda? 

 tendincce 



Papillary muscle 



Longitudinal section of leaflet of 

 tricuspid valve. / , 20. 



