336 THE CIRCULATION OF THE BLOOD. [CHAP. XXVIII. 



continuous with the external tunic of areolar tissue, which invests 

 each of the large arterial and venous trunks that connect them- 

 selves with the heart. This bag serves to fix the position of the 

 heart, and to prevent any sudden or extensive displacement, which 

 might interfere with its proper action. It consists almost exclusively 

 of white fibrous tissue. 



Within this fibrous bag is a serous membrane, the serous peri- 

 cardium, which resembles in all points of arrangement and struc- 

 ture the other membranes of its class. One portion of it invests 

 the internal surface of the fibrous pericardium, while the other 

 covers the heart, the line of reflection passing over the great vessels 

 at the base of the heart. 



The cavities of the heart are lined by the endocardium, a mem- 

 brane continuous with and closely similar to the lining mem- 

 brane of arteries and veins. It consists of a layer of epithelium 

 placed on a stratum of fine fibres which exhibit minute wavings. 

 The epithelium appears to be extremely delicate, but the same 

 in all its characters as that of the blood-vessels. It is so deli- 

 cate that to be seen satisfactorily it must be examined in animals 



Fig. 196 Fig. 197. 



t^W| 



x te 



Epithelium from the left auricle of the Epithelium from the left 

 horse (magnified 200 diams.) showing the ventricle of the horse (mag- 

 two forms of particles, the round and the nified 200 diams.) 

 pointed. 



just killed. We observe two forms of epithelial particles, one 

 soft, rounded and globular, the other somewhat compressed and 

 drawn out at opposite poles into pointed or fibre-like processes. 

 (Fig. 186). It is difficult to determine the precise relative position 

 of these two forms of epithelium ; but it seems probable that the 

 pointed processes are the more deeply seated, and are in immediate 

 contact with the subjacent fibrous layer, which here corresponds 

 to the basement membrane beneath the epithelium of serous and 

 mucous membranes. 



The endocardium of the left auricle, and of the septum and 

 auricular appendage of the right auricle, is thicker and denser than 



