HISTOLOGY 91 



cardium. The myocardium is composed of cardiac muscle fibers which are 

 arranged in spiral sheets. The myocardium of the left ventricle is consider- 

 ably thicker than the right. Both atria have very thin muscle walls. The 

 outermost layer is the epicardium. It is composed of a thin layer of connec- 

 tive tissue, covered by a single layer of mesothelial cells. A transparent, 

 serous membrane, the pericardium, encloses the heart and the proximal 

 portions of the great vessels. The heart lies in the pericardial cavity. 



The right atrium communicates with the right ventricle by the tricuspid 

 valve; the left atrium with the left ventricle by the mitral or bicuspid valve. 

 The valves are composed of folds of the endocardium. They are covered 

 on both sides by endotheHum and contain some connective tissue. They are 

 attached by thin strands, the chordae tendineae, to the projections of the 

 papillary muscles of the ventricular walls. 



According to Ohmori (74), the atrio-ventricular conducting system as 

 described by Tawara exists in the heart of the mouse. 



Arterial blood leaves the left ventricle through the aorta. The opening 

 is guarded by the semilunar aortic valve. From the right ventricle the 

 pulmonary artery originates, guarded by the pulmonary valve, and carries 

 blood to the lungs. From the lungs oxygenated blood is transported to the 

 left atrium through the pulmonary veins. The superior and inferior venae 

 cavae bring venous blood into the right atrium. 



The coronary arteries which branch off from the root of the aorta supply 

 the tissues of the heart with blood. Capillaries are numerous among the 

 heart muscle fibers. 



Lymphatic capillaries and vessels. — The walls of lymphatic capillaries 

 are formed of a single layer of large, flat, polygonal, endothelial cells. The 

 lumina are irregular; dilations and constrictions occur frequently. The 

 capillaries form many branches, some of which end blindly while others 

 anastomose. The lymphatic vessels have thicker walls consisting of, in 

 addition to the endotheHum, collagenous bundles, elastic fibers, and smooth 

 muscle fibers. In the larger lymph vessels an intima, media and adventitia 

 can be distinguished. The intima is formed of endothelium, and a thin layer 

 of elastic fibers, the media of circularly arranged smooth muscle fibers, while 

 the well developed adventitia is composed of collagenous and elastic fibers 

 and smooth muscle bundles. The paired valves are similar to those of veins 

 and consist of folds of the intima. In the largest lymphatic vessel, the 

 thoracic duct, the division of the three parts of the wall is very indistinct. 

 Below the endothelium the collagenous and elastic elements form an inner 

 elastic membrane from which fibers project and mingle with the smooth 



