THE CHAMBERS OF THE HEART. 743 
The Eustachian valve (valvula venee cave inferioris) is a thin and sometimes 
fenestrated fold of endocardium and sub-endocardial tissue, which extends from 
the anterior and lower margin of the orifice of the inferior vena cava to the 
anterior part of the annulus ovalis. Varying very much in size, it is usually of 
falciform shape, its apex being attached to the annulus and its base to the margin 
of the inferior caval orifice. It is an important structure in the feetus, directing 
the blood from the inferior vena cava through the foramen ovale into the left 
auricle. 
The Thebesian valve (valvula sinus coronarii) is usually a single fold of endo- 
cardium which is placed at the orifice of the coronary sinus; occasionally it con- 
sists of two cusps, It is almost invariably incompetent. 
The left auricle (atrium sinistrum) is in relation behind with the descending 
thoracic aorta and the cesophagus. Below and in front it is continuous with the 
left ventricle. Its antero-superior surface is concave, and lies in close relation to 
the roots of the ascending aorta, the pulmonary artery, and the left coronary 
artery. Its right side, formed by the interauricular septum, is directed forwards 
and to the right. Its left side forms a very small portion of the left margin of 
the heart, and from its junction with the antero-superior surface the long and narrow 
“Conus arteriosus 
Left anterior cusp of__ 
pulmonary valve 
Right anterior cusp of 
pulmonary valve 
Left posterior cusp 
of pulmonary valve 
~~ Right coronary artery 
___Anterior cusp of aortic 
Left posterior cusp MEINE 
of aortic valve 
Left coronary artery 
____Right posterior cusp of 
aortic valve 
Anterior (infundibular) 
cusp of tricuspid valve 
Right (marginal) cusp 
of tricuspid valve 
Posterior (septal) cusp 
of tricuspid valve 
mitral valve 
U 
a 
Posterior cusp of 
mitral valve 
rentricle 
Hott peutricls -—-Right ventricle 
Fic. 549.—THE BAsES OF THE VENTRICLES OF THE HEART, showing the auriculo-ventricular, aortic, 
and pulmonary orifices and their valves. 
auricular appendix is prolonged forwards round the left side of the ascending 
portion of the aorta and the trunk of the pulmonary artery. 
The four pulmonary veins enter the upper part of the posterior surface, two on 
each side. 
The interior of the left auricle is lined with endocardium, and its walls are 
smooth, except in the auricular appendix where musculi pectinati are present, and 
on the septum, in a position corresponding with the upper part of the fossa ovalis 
on the right side, where there are several musculo-fibrous bundles radiating for- 
wards and upwards. These septal bundles are separated at their bases by small 
semilunar depressions, in the largest of which remains of the foramen ovale may 
be found. Foramina Thebesu, and the apertures of vense minimi cordis, are scat- 
tered irregularly over.the inner aspect, whilst in the antero-inferior boundary is the 
auriculo-ventricular aperture. This is oval in form; its long axis is placed 
obliquely from before backwards, and from left to right, and is capable of admitting 
two fingers. It is guarded by a valve formed of two large cusps, and is known as 
the mitral orifice. 
Ventricles—The ventricular portion of the heart is conical and somewhat 
flattened. The base, directed upwards and backwards, is partly continuous with 
the auricular portion and partly free. It is perforated by four orifices, the two 
auriculo-ventricular, the aortic, and the pulmonary. The auriculo-ventricular 
