Action of the Heart. 513 



auricle and ventricle. The whole heart of any person is about the size 

 of his closed fist; according to Huxley. Each of the four cavities has 

 the same capacity of from four to six cubic inches. Its whole weight 

 in the adult is 8 to 9 ounces. The blood from the veins of the general 

 system enters the right auricle, which, as soon as it is full, rontnicts and 



Fia. 240. Heart and large vessels seen 



from the front. 

 1. Right ventricle. 

 2. Left ventricle. 



3. Root of pulmonary artery cut short 



4, 4 .Arch of the aorta. 



4 .The descending aorta. 



6. The appendix and front part of right 

 auricle. 



6. Same, of the left auricle. ' 



7, 7 .Innominate veins joining to form 

 the vena cava superior which extends 

 from 7 downwards to the auricle. 



8. Inferior vena cava below the dia- 

 phragm. 



9. One of the large hepatic veins. 



-K +. Left coronary artery. The right 

 coronary artery is seen to emerge from the 

 aorta between 3 and 5. ( R. Quain.) 



FIG. 241. Heart and great vessels seen 

 from behind. 



1. Right ventricle. 



2. Left ventricle. 



3. Right pulmonary artery. 



3'. Its branches passing into the root of 

 the right lung. 



3". Same of the left. 



4 .Arch of the aorta. 



4". Descending aorta. 



5. Right auricle. 



6. Is on the division between the right 

 and left auricles. 



7. Superior vena cava. 



7 .Left innominate vein. 



8. Inferior vena cava. 



9. Right hepatic vein. 



10, 11, 12. Right pulmonary veins. 



13, 14. Left pulmonary veins. 



+, +. Branches of right and left coro- 

 ( R. Quain.) 



nary arteries. 



drives the blood Into the right ventricle through a passage way, which is 

 closed by the tricuspid ( three-pointed ) valve. This valve is composed 

 of three triangular flaps, the bases of which are attached in a ring to 

 the inside walls of the heart, while the long pointed ends project into 

 the ventricle. These points give way before the incoming blood from 

 the auricle. As soon as the right ventricle is full it contracts, and the 

 pressure instantly closes the tricuspid valve, which is prevented from 

 becoming inverted into the auricle by a number of fine, strong, tendi- 

 nous cords, which connect the points of the flaps with portions of the 

 walls of the ventricle. They are the chordae tendince. The blood thus 

 forced out of the right ventricle goes up the pulmonary artery to the 

 lungs. The ventricle then dilates again, at the same time receiving an- 



