TOPOGRAPHY OF THE THORAX AND ABDOMEN 29 



XXVIII, XXX and XXXI. The tricuspid orifice is some- 

 what enlarged on account of the distention of the right 

 auricle. It is oblong in outline, the long axis — superoin- 

 ferior — being about 4.5 cm. and the short axis about 

 3.5 cm. in length. The long axis coincides roughly with 

 that of the heart, and the plane of the orifice forms an 

 angle of about 45 degrees with the midplane (Plates 

 X and XI). The blood, in passing from the auricle 

 into the ventricle, has a direction forward, to the left and 

 a little downward. The orifice lies beneath the left half 

 of the sternum extending from .5 cm. to the right of the 

 midline, to the left border of the sternum. It extends from 

 the level of the middle of the third to the upper part of the 

 fifth sternochondral articulation or from the level of the 

 upper border of the seventh to the lower border of the 

 eighth thoracic vertebra. 



The pulmonary orifice is behind the lower part of the 

 second costal cartilage and the upper half of the second 

 intercostal space at the left margin of the sternum. The 

 plane of the orifice forms an angle of about 45 degrees with 

 the coronal plane, so that the blood in passing into the 

 pulmonary artery takes a direction upward, backward and 

 a little to the left. The orifice is guarded by three semi- 

 lunar valves, one of which is anterior, one internal and one 

 posterior. 



The mitral orifice is small and relatively wider than 

 the tricuspid. Its long axis is nearly vertical, being about 

 4 cm. in length, while its short axis is about 3.5 cm. in length 

 The blood takes a direction downward, forward and to the 

 left in passing from the auricle into the ventricle. This 

 orifice is located almost entirely to the left of the sternum 

 behind the third costal cartilage, third intercostal space, 

 and upper half of the fourth costal cartilage at the level of 



