122 THORAX 



inferior vena cava and the coronary sinus ; and into the left atrium through 

 the four pulmonary veins. The atrial contraction commences with the 

 contraction of the circular fibres which surround the mouths of the veins 

 entering the atria, and thus the blood is prevented from passing back 

 into the veins. As the contraction extends to the general fibres of the atria 

 the blood is forced onwards iato the ventricles, which become distended. 

 Then the ventricular contraction commences, the atrio-ventricular valves 

 close, and, as the contraction proceeds, the blood is driven out of the 

 ventricles through the arterial orifices, that in the right ventricle being 

 ejected into the pulmonary artery, and that in the left ventricle into the aorta. 



When the ventricular contraction is completed the period of diastole 

 commences ; and, as long as the heart remains alive, the cycle is repeated. 



The work of the atria is merely to force the blood through the widely 

 open atrio-ventricular orifices into the ventricles and to expand the dilating 

 walls of the ventricles. For that purpose no great force is required, there- 

 fore the walls of the atria are thin. The work of the ventricles is much 

 more severe, therefore their walls are thicker. The right ventricle, however, 

 has only to exert sufficient force to drive the blood through the lungs to the 

 left atrium, that is, through a comparatively short distance and against a 

 comparatively small resistance ; therefore its walls are thin as compared 

 with the walls of the left ventricle, which have 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, 13 mm. (half an inch) from the left border of the 

 sternum, and ending at the upper border of the third right 

 costal cartilage, about 13 mm. (half an inch) from the right 

 border of the sternum. The line so drawn will indicate 

 the position of the upper border of the heart, which is 

 formed by the upper ends of the atria. (2) A line from the 

 upper border of the third right costal cartilage to the sixth 

 right costal cartilage. That line should commence and end 

 about 13 mm. (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, about 

 88 mm. (three and a half inches) from the median plane. 

 That 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 constitution only in the region of the apex. 



