92 PERICARDIUM; ENDOCARDIUM; VALVES. 



elastic fibers, and is covered on its free surface with a single layer of irregular 

 polygonal, flat, endothelial cells. A rich network of lymph-vessels lies within the 

 pericardium itself, as well as more deeply toward the muscle-mass of the heart. 

 Stomata are wanting in both layers of the pericardium. In the subserous tissue 

 of the pericardium, especially in the sulci for the coronary vessels, are deposits 

 of fat, and lymphatics. 



The endocardium presents all of the characteristics of a vessel-wall. Facing 

 the cavity of the heart, there is first a single layer of flat, polygonal, nucleated 

 endothelial cells. Then there comes, as the true groundwork of the whole mem- 

 brane, a layer of delicate elastic fibers (more marked in the auricles, and even 

 forming a f enestrated membrane) , in the midst of which but little connective 

 tissue occurs. The latter, much more loosely arranged and intermixed with elas- 

 tic fibers, is present in larger amount toward the heart-muscle. Scattered 

 bundles of unstriated muscular fibers, usually arranged longitudinally, are found 

 between the elastic elements (in smaller amount in the auricles) . These obviously 

 have the task of combating the pressure and the tension exerted on the endocar- 

 dium during the cardiac contraction; for wherever throughout the body a wall 

 composed of soft parts is exposed to repeated high pressure muscular elements 

 are found, and never elastic tissue alone. The endocardium is non- vascular. 



The valves both the arterial (semilunar) and the venous (mitral and tricuspid) 

 also are a part of the endocardium. The venous and arterial orifices on the 

 right side are separated from each other in the wall of the ventricle, while the 

 two orifices on the left are united into a single large opening. The valves are 

 attached to their basal margins by means of resistant fibrous rings composed of 

 connective-tissue and elastic fibers. They consist of two layers: (i) The fibrous, 

 which is a direct continuation of the fibrous ring, and (2) a layer of elastic elements. 

 The elastic layer of the auriculo-ventricular valves is a direct prolongation of the 

 endocardium of the auricle, and is therefore directed toward that cavity. At 

 their bases the valves are united by their adjacent margins. The tendinous cords 

 are inserted on the free margin and on the under surface of the valves. The 

 semilunar valves possess a thin, elastic layer, thickened at their base and turned 

 toward the arteries. 



The auriculo-ventricular valves contain also striated muscle-fibers. Radiating 

 fibers, arising from the auricles, extend into the valves, and it is their function 

 in part to retract the valves toward their bases during the time of auricular systole, 

 and thus to enlarge the passage-way for the flow of blood into the ventricles. 

 Paladino describes still other longitudinal fibers derived from the ventricles. 

 Besides these, there is directed rather [toward the ventricular aspect, a con- 

 centric muscular layer, following the basal attachment of the valves, which 

 appears to have a sphincter-like action drawing the bases of the valves together 

 during the period of ventricular contraction when the valves are under tension, 

 and thus preventing excessive distention. The larger of the tendinous cords also 

 contain striated muscle-fibers; and the Thebesian and Eustachian valves like- 

 wise contain a delicate muscular network. 



The name "Purkinje's fibers" has been applied to a grayish network of 

 muscular elements found in mammals and in birds chiefly beneath the endocar- 

 dium of the ventricle, but occurring also in the muscular mass itself. These 

 appear to represent a stage of embryonal development (on account of the partial 

 striation) . They are absent in man and in the lower vertebrates. 



Blood-vessels occur in the auriculo-ventricular valves in considerable number 

 only where there are muscle-fibers. In children delicate vessels extend to the 

 free margin of the valve. The semilunar valves are devoid of blood-vessels except 

 under pathological conditions. A network of lymphatics extends from the endo- 

 cardium to the middle of the valves. 



Weight and Size of the Heart. According to W. Muller, the weight of the 

 heart in children and in older persons having a body- weight up to 40 kilos, is 5 

 grams for every kilo of body-weight ; in individuals having a body-weight of from 

 50 to 90 kilos, the proportion is 4 grams of heart for each kilo; in individuals 

 having a body-weight of 100 kilos, 3.5 grams of heart for each kilo of body-weight. 

 The auricles become stronger with increasing age. The right ventricle weighs 

 half as much as the left. In man the heart weighs 309 grams; in woman, 274 

 grams. Blosfeld and Dieberg found the heart in man to weigh 346 grams; in 

 woman, from 310 to 340 grams. The thickness of the left ventricle in man aver- 

 ages 11.4 mm.; in woman, 10.15 mm.; the thickness of the right ventricle, 4.1 

 and 3.6 mm. respectively. 



