ANGIOLOGY 207 



The heart is surrounded by a sero-fibrous sac, the pericardium. 

 This sac adheres to the cervical air reservoirs anteriorly and to the 

 diaphragmatic septum posteriorly. It is composed of two mem- 

 branous layers: the parietal, external, dense, and fibrous; and the 

 visceral, internal, and serous. The pericardial sac has no direct 

 attachment to the heart, except at the upper extremity where it 

 surrounds the large vessels emerging from it. The serous layer is 

 reflected over the outer portion of the heart, where it is called the 

 epicardium. The function of the pericardium is to prevent friction 

 during the beating of the heart. It contains a small amount of 

 serous fluid for perfect lubrication. This fluid is called the liquor 

 pericardii. 



Internally the heart has four cavities: two auricles and two 

 ventricles. The right ventricle is more crescent-shaped than in 

 sohpedes, and in a manner envelops the left ventricle in front and to 

 the right, though it does not reach the point of the heart. The 

 right auricle is larger than the left. The auricula-ventricular valve 

 is not trictispid as in mammals. This valve instead of being formed 

 as usual by a membranous curtain, with margins retained by cords 

 fixed to the walls of the ventricles, is composed of a wide muscular 

 leaf, which appears to be a portion of the inner wall of the ventricle 

 detached from the interventricular septum. This septum is convex; 

 and the auriculo-ventricular orifice is an obhque sUt situated between 

 it and the muscular valve in question; so that, when the heart wall 

 contracts at the systole, the valve is applied against this septum and 

 closes the passage. The bicuspid, or auriculo-ventricular valve of 

 the left side usually has two segments, though occasionally there 

 may be three. The fossa ovalis is a depression behind the posterior 

 semi-lunar valve in the septum of the heart. The membranous sep- 

 tum closing the foramen ovale is complete and strong but thin and 

 transparent. The right auricle receives the blood from the two 

 venje cavas coming from the anterior extremity, and from the 

 posterior vena cava. These empty into a sinus. The left auricle 

 has two vessels, the pulmonary veins which bring blood to it from 

 the lungs. 



Structure of the Heart. — The heart is lined by a serous membrane, 

 the endocardium, which is a continuation of the endothelium of the 

 blood-vessels. There are a few muscular pillars in the inner wall, 

 called the columnce carnce. To give the heart its pumping power, 

 it is made up of contractile tissue, a speciahzed kind of muscle 



