2GS CIRCULATORY APPARATUS. 



The endocardium is a far more delicate membrane than the 

 internal coat of the aorta. This is especially well seen on the 

 inner surface of the ventricles, where it appears simplj as a 

 lustrous surface, limiting the muscular layer internally, but not 

 in any way affecting either its colour or its modelling. Where it 

 forms duplicatures in which strength is especially required — sc. 

 the valves and chordae tendinejB — it becomes thicker. In the 

 valves indeed we find the endocardium made up of several dis- 

 tinct layers. Thus the lamella of each valve-flap which is turned 

 away from the blood-current contains a larger proportion of 

 elastic fibres than that which is turned towards it, and between 

 the two lamella}, at least in tlie mitral and tricuspid valves, there 

 is a thin layer of loose connective tissue in which vessels run 

 nearly as far as the edge of the valve. 



The myocardium is the thickest of the three layers of which 

 the heart's wall consists, as might be anticipated from its func- 

 tional importance ; its structure affords a solution of the difficult 

 problem, how to enclose a cavity with striped muscular fibre. 

 The fibres anastomose with one another at acute angles, forming 

 fenestrated membranes ; and these, lying one upon another in 

 massive strata, and firmly united among themselves, enclose the 

 cavity (c£ § 235, fig. 85). 



The adventitia reappears as a serous sac, the pericardium, 

 whose visceral layer invests the outer surface of the heart with 

 a membrane quite as thin as the endocardium upon its inner 

 surface. It nevertheless contains vessels of its own, although 

 they anastomose at many points with those of the muscular 

 substance. 



The above remarks all tend to show that the three layers of 

 which the Avail of the heart consists, arc more specialised, more 

 independent, than the corresponding structures in the arteries 

 and veins ; and since this increased specialisation makes itself 

 felt in the diseases to which they are liable, it must be borne In 

 mind throughout the whole of the ensuing discussion. Accord- 

 ingly, the pathological histology of the myocardium and endocar- 

 dium will be treated successively, that of the pericardium being 

 relegated to the general chapter which will be devoted to the 

 morbid anatomy of serous membranes. 



