104 THE HUMAN BODY. 



leaving as the only trace of its existence a depression called 

 the fossa ovalis. 



The superior and inferior vena cava open into the right 

 auricle, and at the orifice of the latter is the Eustachian 

 valve. The orifices of the right and left pulmonary veins 

 are in the left auricle. 



The opening by which the auricles and ventricles commu- 

 nicate with each other is called the auriculo-ventricular 

 opening. These orifices are furnished with valves; that on 

 the right is called the tricuspid valve, from the three angles 

 which are formed by its leaves; and that on the left is called 

 the mitral valve, from the slight resemblance which it bears to 

 a bishop's mitre. 



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

 very fine, smooth membrane, which has been compared to 

 the serous membranes. These cavities present numerous 

 inequalities, which result from the projection of the bundles 

 of muscular fibre which point in every direction. In the ven- 

 tricles these fascicles form fleshy columns (columna camece), 

 disposed in a net-work running from one point of the walls to 

 another, and several which take part in the movement of 

 the valves, send out to these valves a crowd of little tendons. 

 The walls of the left ventricle are much thicker and more 

 resistant than those of the right ventricle. 



Pericardium. This is the term applied to the covering 

 which envelops the heart; it is a sac composed of two layers, 

 a fibrous membrane on the outside, and a serous membrane 

 on the inside. This last covers the external surface of the 

 heart, and is reflected back upon itself in order to form, like 

 all the membranes of this nature, a sac without an opening. 

 The heart is thus covered by the pericardial sac, but not 

 contained inside its cavity. A correct idea may be formed 

 of the disposition of the pericardium around the heart by 

 recalling a very common and very convenient, though now 

 discarded head-dress, the cotton night-cap. The pericardium 

 incloses the heart exactly as this cap covered our forefathers' 

 heads. 



Arteries. The vessels which carry the blood from the 

 right ventricle to the lungs, and from the left ventricle to the 



