744 THE VASCULAR SYSTEM. 
orifices are placed one on each side below and behind; in front and between 
them is the aortic orifice, whilst the orifice of the pulmonary artery is still farther 
forward, and slightly to the left of the aortic. 
In the trian 1gle (trigona fibrosa) between the auriculo-ventricular and the aortic 
orifices is embedded the central fibro- cartilage, a mass of fibro-cartilaginous tissue 
which is the representative of the os cordis of the ox. It is continuous with the 
upper part of the interventricular septum, and with fibrous rings which surround 
the apertures at the bases of the ventricles. 
The apex of the ventricles forms the apex of the heart. The inferior surface 
and the antero-superior surface constitute respectively the greater portions of the 
corresponding surfaces of the heart; the former rests upon the diaphragm, whilst 
the latter is directed upwards and forwards towards the sternum and the costal 
cartilages of the left side. 
The right margin, which is thin, forms the horizontal portion of the right 
margin of the heart : : Fat the left margin, which is thick and rounded, forms aianoen 
the whole of the left margin of the heart. 
The ventricular portion of the heart is divided into right and left chambers. 
The interventricular septum (septum ventriculorum) is placed obliquely, with one 
surface directed forwards and to the right, and the other backwards and to the 
left; it bulges into the right ventricle, and its lower margin les to the right of 
the “apex of the heart, which is, therefore, formed entirely by the left ventricle. 
The margins of the septum are indicated on the surface by anterior and inferior 
interventricular sulci. 
The right ventricle (ventriculus dexter) is triangular in form. Its base is 
directed upwards and to the right, and in the greater part of its extent it is con- 
tinuous with the right auricle, with 
which it communicates by theauriculo- 
ventricular orifice; but its left and 
anterior angle projects in front of the 
auricle, and gives origin to the pul- 
monary artery. Its inferior wall rests 
upon the diaphragm. The antero- 
superior wall les behind the lower 
part of the left half of the sternum 
and the cartilages of the fourth, fifth, 
and sixth ribs of the left side. The 
left or septal wall, which is directed 
backwards and to the left, bulges into 
its interior, and on this account the 
transverse section of the cavity has 
a semilunar outline. The cavity itself 
is separable mto two parts—a right or 
posterior portion, the body, into which 
the auriculo-ventricular orifice opens, 
and a left or anterior portion, the in- 
fundibulum or conus arteriosus, which 
terminates in the pulmonary artery. 
The two sections of the cavity are 
separated above by a thick fold of 
muscle, but otherwise they are quite 
continuous with each other. 
Fic. 550,—THE RELATIONS OF THE HEART AND THE The right auriculo-ventricular orifice 
AURICULO-VENTRICULAR, AORTIC, AND PULMONARY -_ | , F : 
ORIFICES TO THE ANTERIOR THORACIC WALL, ts guarded bya tricuspid valve (valvula 
tricuspidalis). The three cusps of this 
I to VII, Costal cartilages. M, Mitral orifice. 
A, Aortic orifice. Salone) orifice: valve are a right or marginal (cuspis 
Ao, Aorta. A, Right auricle. 
C, Clavicle. RV, Right ventricle. medialis), a lett or infundabular (cuspis 
LA, Left auricle. SVec, Superior vena cava. 7 2 
LY, Left ventricle. T, Tricuspid orifice. anterior )s W hich inter venes betw eel 
the auriculo-ventricular orifice and 
the infundibulum, and a posterior or septal (cuspis posterior). Hach cusp 
ale 
