30 UNIVERSITY OF MISSOURI STUDIES 



the seventh thoracic vertebra. Its center is about 1.5 cm. 

 above, 3.5 cm. behind and 2 cm. to the left of the center of 

 the tricuspid orifice. 



The aortic orifice lies behind the left half of the ster- 

 num at the level of the upper border of the third costal 

 cartilage and opposite the middle of the sixth thoracic 

 vertebra. The plane of this orifice is more nearly hor- 

 izontal than that of any of the other openings. It is 

 about 3 cm. below and 3 cm. internal to the pulmonary 

 orifice. The blood in passing into the aorta takes a direc- 

 tion upward, to the right and a little backward. The semi- 

 lunar valves which guard the opening are one anterior and 

 two posterior. The mesial posterior cusp is a little more 

 anteriorly placed than the lateral (Plate IX). 



The heart enters into relations with the surrounding 

 organs through the pericardium which surrounds it and 

 which will be considered as a part of the heart in this 

 paragraph. The right auricle is separated from the ante- 

 rior thoracic wall by the thin anterior part of the right 

 lung. The right ventricle is in contact for the most part 

 with the gladiolus throughout the entire extent of that 

 bone, and with the second, third, fourth and fifth left costal 

 cartilages and the intercostal muscles. Nearly half of the 

 narrow strip of the left ventricle seen in the anterior pro- 

 jection is in direct contact with the ribs and intercostal 

 muscles. The remainder of the ventricle together with the 

 left auricle, is covered anteriorly by the anterior part of 

 the left lung. Thus the greater part of the anterior sur- 

 face of the heart is not covered by the lungs. This un- 

 covered area which is about 10 cm. wide across at the fourth 

 ribs is to be explained by the extreme contraction of the 

 lungs and accompanying distention of the right auricle 

 (Plates X and XI) and by the exclusion of the left lung 



