260 PHYSIOLOGY. 



Fig. 2. 



View of the heart with its several chambers exposed and the ves. 

 sels in connection with them. 1. The superior vena cava. 2. The 

 inferior vena cava. 3. The chamber called the auricle. 4. The right 

 ventricle. 5. The line marking the passage between the two cham- 

 bers, and the points of attachment of one margin of the valve. 6. The 

 septum between the two ventricles. 7. The pulmonary artery, arising 

 from the right ventricle, and dividing at 8 into right and left, for the 

 corresponding lungs. 9. The four pulmonary veins, bringing the blood 

 from the lungs into 10, the left auricle. 11. The left ventricle. 12. 

 The aorta, arising from the left ventricle, and passing down behind the 

 heart, to distribute blood to every part of the system. Thus the blood 

 moves in a double circle, one from the heart to the body, and from the 

 body back to the heart, called the systemic circle ; the other, from 

 the heart to the lung, and from the lung back to the heart, called the 

 pulmonic circle. 



12. The arteries are very strong and elastic, of a yellow- 

 ish white colour Jand have three coats, jthe outer one of which 

 is called cellular^lhe middle, the fibrous coat, and the inter- 

 nal, serous. It is very important that arteries should be 

 elastic and capable of stretching, because, if they were not, 

 every time a limb was broken, the artery would be ruptured, 

 and the person bleed to death. (See Fig. 3.) 



13. JThe veins which return the blood to the heart consti- 

 tute two systems like the arteries ; the one brings all the 

 dark-coloured blood from the head, trunk and limbs, and in- 

 ternal organs, to the right side of the heart, into which it 

 opens by the two great trunks, called the upper and lower 



