THE HEART. 



597 



Fig. 358. 



are prolonged into the veins and arteries, to form the internal tunic of these 

 vessels. At the auriculo-ventricular and arterial openings, they constitute a 

 duplicature for the valves situated there. These valves are, therefore, due to the 

 projection of a circular fold of the endocardium, between the two layers of which is 

 cojinective tissue mixed with elastic fibi-es in the auriculo-ventricular valves, and 

 a layer of connective tissue on each surface of the elastic fibres in the semilunar 

 valves. It is worthy of remark that the elastic fibres of the valves are more 

 abundant in the left than the right side of the heart. 



The endocardium of the right heart has a red tint, which is deepest in the 

 ventricle. In the left heart, this tint is slightly yellow, especially in the walls of 

 the auricle, which may be attributed to the presence of a 

 thin layer of yellow elastic tissue that covers the adherent 

 face of tlie membrane. 



The endocardium consists of two layers : 1. A thin bed 

 of connective tissue, connecting it to the muscular structure, 

 in which elastic fibres are pretty uniformly distributed, as 

 well as smooth muscular fibres, especially in the left heart. 

 2. An endothelium, consisting of a single or double layer 

 of somewhat elongated, polygonal, tesselated nucleated cells. 



2. External serous membrane (epicardium). — This is the 

 visceral lining membrane of the pericardium, the description 

 of which follows 



5. The Pericaedium (Fig. 255, c). 



Preparation. — Place the animal in the seccid position, and remove 

 the sternal ribs by separating the cartilages and luxating their costn- 

 vertebral articulations. This procedure permits the study of the 

 situation and general disposition of the heart and pericardium. But 

 in order more easily to examine tiie reciprocal arrangement of these two 

 parts, it is necessary to extract them from the thoracic cavity by tearing 

 through the sternal insertion of the pericardium. 



The pericardium, or proper serous covering of the heart, is a membranous sac 

 enclosing that organ, fixing it in the thoracic cavity, and favouring its movements 

 by its polished surface. 



This sac is formed by a fibrous layer, within which is spread a serous 

 membrane, divided into two parts — one parietal, the other visceral. 



The fibrous layer of the pericardium presents somewhat the general form of 

 the heart. Its internal surface is covered by the parietal portion of the serous 

 membrane. The external surface corresponds to the two layers of the mediastinum. 

 Its summit (or apex), depressed on each side, and elongated from before to behind, 

 is firmly attached to the superior face of the sternum, from the fourth rib to the 

 origin of the xiphoid cartilage. By its base, it is fixed to the large vessels going 

 to and leaving the heart, where it is continuous with their connective-tissue 

 sheath, and where it sends some fibres to the longus colli. 



The serous membrane of the pericardium has been well compared by Bichat to 

 a cotton nightcap, the external part of which would represent the parietal layer, 

 and the inverted part the visceral portion of the membrane. The parietal layer 

 adheres in the most intimate manner to the internal face of the fibrous tunic, 

 and is reflected to form the visceral portion, around the pulmonary arteries and 

 the aorta for a certain distance from their origin, and on the pulmonaiy veins. 

 The visceral layer envelops in common the two arterial trunks, covers a small 



EPITHELIUM OF THE 

 EXDOCARDIUJI. 



, Nucleated cells as- 

 suming the fusiform 

 figure; 2, polygonal 

 nucleated cells. 



