DESCRIPTION OF THE HEART. 633 



back, its orifice being protected by a thin membranous valve, the coro- 

 nary valve, or valve of Thelesius; besides this, there are numerous 

 apertures of small veins belonging to the heart, and certain recesses in 

 the auricular walls. Upon the septum, between this and the left 

 auricle, is an oval depression, the fossa ovalis^ bounded above, and at 

 the sides, by a margin named the annulus ovalis. The fossa ovalis is 

 the vestige of an opening, the foramen ovale, which exists before birth, 

 then permitting the blood to pass from the right into the left auricle: 

 sometimes the foramen ovale is not entirely obliterated, in that case, 

 a small valved aperture leading obliquely, beneath the annulus ovalis, 

 into the left auricle. Attached to the anterior margin of the orifice of 

 the inferior vena cava is a thin membranous semilunar fold, called 

 the Eustachian valve (Fig. 105), the free concave border of which is 



Fig. 105. 



\ 



Fig. 105. Diagram of the heart and great bloodvessels. The right cavities of the heart, or right auricle 

 and ventricle, and the pulmonary artery, are supposed to be laid open. 1, the superior vena cava; 2, in- 

 ferior vena cava; 3, right auricle, laid open, showing the orifice of the superior and inferior cavie. the latter 

 guarded by the Eustachian valve; 4, right ventricle, laid open, showing the anterior segment of the tricns- 

 pid valve, its chordae tending, and musculi papillares. The thin walls of the auricle, and the thicker walls 

 of the ventricle, are seen on their sections; 5, pulmonary artery, laid open, to show parts of two of its semi- 

 lunar valves *; 7, part of the left auricle: the pulmonary veins are not represented, being concealed at 

 the back of the heart; 8, the left ventricle; 9, the aorta, giving off branches to the head and upper limbs, 

 and arching down to form the abdominal artery, which supplies branches to the rest of the body. The 

 arrows show the course of the blood from the veins, 1. 2, to the right auricle; 3, through the auriculo- 

 ventricular opening into the right ventricle, 4, and thence along the pulmonary artery, 5. 



turned upwards and to the left ; it is often small, frequently perforated, 

 and sometimes wanting. Before birth, it is large, and of great im- 

 portance in directing the course of the blood. Lastly, in front and 

 to the left of the opening of the inferior cava, is the large aperture 

 leading into the right ventricle, named the right auriculo-ventricular 

 opening. 



The right ventricle, 4, forms a somewhat conical cavity, shut off 

 from the left ventricle by the thick interventricular septum. At its 



