THE BACK. 199 



its smaller to tlie left. On examining tlie inner surface of 

 this vessel three saccular dilatations (sinus Valsalvce) of its 

 walls may be noticed behind the valves, serving to accom- 

 modate them during the rush of blood from the ventricle, 

 and to allow blood to collect behind the flaps in sufficient 

 quantity when this rush ceases to throw the valves into 

 position and thus prevent regurgitation of blood. The 

 pulmonary arteries divide and subdivide in the lungs, 

 and finally terminate in capillaries in which the blood is 

 oxygenated, and from which it is carried by the pulmonary 

 veins to the left auricle of the heart. The pulmonary 

 veins unite and reunite until by (generally) four openings 

 they pour their contents into the heart ; they never leave 

 the lung substance, for it is here also attached to the heart 

 substance. These veins have no valves. The left auricle 

 very much resembles the right, from which, however, it 

 differs in the greater amount of yellow elastic tissue lining 

 it, and in presenting more marked musculi pectinati and 

 foramina Thebesii,but no fossa ovalis,annulus ovalis, tuber- 

 crum Loweri, or Thebesian valve. Its auriculo -ventricular 

 opening too is guarded by a stout valve consisting of but 

 two flaps, the hicuspid or mitral valve. It leads into the 

 left ventricle, similar in structure and figure to the right 

 ventricle, but with only two musculi papillares, both of 

 which are attached to the outer wall of the cavity. Its 

 moderator bands generally contain muscular fibres, and its 

 walls consist of very thick muscular substance. This com- 

 partment, however, at the extreme inferior part, where it 

 alone forms the apex of the heart, consists of little more 

 than a double layer of serous membrane, the endo- 

 cardium lying in contact with the pericardium. The thick- 

 ness of the muscular wall is related to the force required 

 in driving the blood through the system when the elasticity 

 of the coats of many large and strong vessels has to be 

 temporarily counteracted. The blood from this cavity 

 passes in an upward direction into the aorta, which is 

 situated on the right side of the pulmonary artery, to 

 which one of its primary divisions (posterior aorta) is 

 connected by an elastic ligament. The opening into this 

 vessel, like that of the pulmonary artery, presents semi- 

 lunar valves, but they are larger, stronger, and present 

 more marked corpora arantii ; while the sinus Valsalvse 

 into which they fit are deeper than those of the pulmonary 



