MINUTE ANATOMY OF THE HEART. 251 



The connective tissue is closely connected with the mus- 

 cular substance of the heart, and presents at some spots a remark- 

 able condensation ; it is arranged in well-marked layers this 

 is particularly the case in the so-called fibrous rings at the car- 

 diac orifices, and in a lesser degree at the apices of the papillary 

 muscles, both being points which constitute the origin, or 

 perhaps the termination, of muscular fasciculi. The fibrous 

 rings are composed of very strong fibrous tissue, traversed 

 by exceedingly fine elastic fibres, and sometimes assume to 

 some extent the character of cartilage, the appearances presented 

 resembling those found in true cartilage, at its point of tran- 

 sition into perichondrium. To these differences, which are 

 by no means essential, the somewhat discordant statements and 

 descriptions made by various authors may be ascribed. At the 

 cardiac orifices the fibrous tissue enters into the formation of 

 the valves, and in the papillary muscles it passes immediately 

 into the tissue of the chordae tendinese, though always sharply 

 separated from the tissue of the endocardium. 



The endocardium forms a membranous lining to the cavities 

 of the heart, but is not everywhere of equal thickness. It par- 

 ticipates in the construction of the valves, and is composed of 

 several layers. Its proper basis is formed of an elastic layer, 

 which contains networks of elastic fibres developed to a variable 

 extent, with a corresponding variation in the quantity of con- 

 nective tissue. The external layer is the loosest in texture. Its 

 internal surface is lined by a layer of nucleated polygonal cells, 

 resting upon a peculiar close-textured lamella of elastic fibres, 

 which constitutes the endothelium of the cardiac cavities. 



It may be added that the simple elastic lamina usually 

 adheres closely to the muscular wall itself by means of a layer 

 of connective tissue, whilst the muscular tissue aids in the 

 formation of the endocardium by giving off to it both smooth 

 and transversely striated fibres. 



The smooth muscle cells are introduced between the elastic 

 lamellae, but do not form a continuous layer, being arranged in 

 separate bands, which vary in size, and sometimes attain a thick- 

 ness of O10 millimeters. The several layers of the muscle cells 

 in these fasciculi do not all pursue the same direction, though 

 they generally appear to be divided transversely when the section 



U 





