40 CIRCULATION OF THE BLOOD. 



37. This vessel divides into two branches, one going to the 

 right and the other to the left, to enter the two corresponding lungs, 

 and are divided into almost an infinity of branches, which are 

 spread over the surface of the little membranous cells of these 

 organs. 



38. The capillary vessels by which the pulmonary arteries ter- 

 minate, give rise to veins, which unite together and finally form 

 two large vessels, called pulmonary veins, which empty into the 

 Jefr. auricle of the heart. ( page 36, fiy. 5.) 



39. Consequently, the pulmonary veins receive the venous 

 blood, which was brought to the lungs by the pulmonary artery, 

 and which has now become arterial, by the effect produced on it, 

 by contact with the air in the interior of these organs ; they carry 

 it back again to the heart and pour it into the left auricle. 



40. Finally, from the left auricle this fluid descends into the left 

 ventricle, whence we have already seen it issue to be distributed 

 to the different parts of the body, through the medium of the aorta 

 and its branches. 



To recapitulate what has just been said, on the route pursued by 

 the blood, in the apparatus .of the circulation in mammiferous 

 animals and birds, we see : 



41. 1st. That the venous blood arrives from all parts of the 

 body by the general system of veins ; 



2d. That from these veins it enters the right auricle of the 

 heart ; 



3rd. That from the right auricle it passes into the right 

 ventricle ; 



4th. That from the right ventricle the venous blood passes 

 through the pulmonary artery to the lungs; 



5th That in the capillary vessels, which form the termina- 

 tion of the pulmonary artery, and commencement of the -pul- 

 monary veins, this liquid is changed into arterial blood ; 



6th. That this arterial blood returns from the lungs, through 

 the pulmonary veins, and enters the left auricle of the heart ; 



7th. That from the left auricle it descends into the ventricle 

 of the same side ; 



8th. That from the left ventricle it is forced into the aorta, 

 by which it is distributed to all parts of the body. 



37. What is the distribution of the pulmonary artery ? 



38. What is the origin of the pulmonary veins 7 Where do they empty ? 



39. What kind of blood do the pulmonary veins convey to the heart? 

 How is the venous, changed into arterial blood 7 



40. What becomes of the blood after it enters the left auricle ? 



41 * Whut is the route pursued by th Hood in the apparatus of the circu- 

 lation ? 



