94 THORAX 



auricle, that is, through a comparatively short distance and against a com- 

 paratively small resistance ; therefore its walls are thin as compared with 

 the walls of the left ventricle, which has to be sufficiently strong to force 

 the blood through the whole of the trunk, the head and neck, and the upper 

 and lower limbs. 



The Topography of the Heart. Before proceeding to the 

 study of the trachea, the dissectors should replace the heart 

 in position and revise their knowledge of its relations to the 

 surface. Its position can be indicated on the anterior wall 

 of the thorax by the following four lines: (i) A line com- 

 mencing at the lower border of the second left costal 

 cartilage, half an inch from the left border of the sternum, 

 and ending at the upper border of the third right costal 

 cartilage, half an inch from the right border of the sternum. 

 This line indicates the position of the upper border of the 

 heart, which is formed by the atria. (2) A line from 

 the upper border of the third right costal cartilage to the 

 sixth right costal cartilage. This line should commence 

 and end half an inch from the border of the sternum, and 

 should be slightly convex to the right. It indicates the 

 right border of the heart, which is formed by the right atrium 

 alone. (3) A line from the sixth right costal cartilage to the 

 apex, which lies behind the fifth left intercostal space three 

 and a half inches from the median plane. This line marks 

 the position of the lower border of the sterno-costal surface, 

 which is formed, in the greater part of its extent, by the 

 right ventricle, the left ventricle entering into its constitu- 

 tion only in the region of the apex. (4) A line from the 

 apex to the lower border of the second left costal cartilage. 

 This line should be convex upwards and to the left; the 

 point of greatest convexity should coincide with the lower 

 border of the fourth left costal arch, and the upper extremity 

 should be situated half an inch from the left margin of the 

 sternum. It marks the position of the left border of the 

 heart, which is formed in four-fifths of its length by the left 

 ventricle and in the remaining fifth by the left atrium. 



A line from the upper border of the sternal end of the 

 third left costal cartilage to the lower border of the sternal 

 end of the sixth right cartilage indicates the anterior part 

 of the coronary sulcus. The points indicating the positions 

 of the arterial and atrio-ventricular orifices must be placed 

 below and to the left of the line of the coronary sulcus in 

 the following order from above downwards ; pulmonary orifice^ 



