PULMONARY CIRCULATION. 101 



lower trunk of the venous system. This trunk ascends the 

 abdomen on the right of the aorta, receiving in its course 

 all the tributary branches, (except those forming the vena 

 portas,) to the diaphragm, through which it passes direct 

 to the right auricle of the heart. This vessel returns all 

 the blood of the inferior half of the body. 



In the superior portion of the body we commence with 

 the sinuses of the brain, which, emerging at the base of 

 the cranium, become the internal jugulars. These descend 

 the neck, and, with the external jugulars, unite with the 

 subclavian at the base of the neck, and form the vena in- 

 nominata, which, with the vena azygos coming together 

 upon the right side, constitute the superior cava. This 

 vessel then descends to the right auricle, entering at its top. 



This brings us to the next circulation in order, the 

 Pulmonary. This is also called the lesser circulation, in 

 contradistinction to the general, styled the greater or sys- 

 temic. 



It occupies a position intermediate between the venous 

 and arterial trunks of the general circulation, by ending 

 the former and commencing the latter. The trunk of this 

 circulation begins in the upper portion of the right ven- 

 tricle of the heart, and after a short distance divides into 

 two primary branches, which go to the lungs; these, in turn, 

 again divide and subdivide into countless numbers, which 

 distribute themselves in the form of a net-work over the air 

 cells, which is called the rete malpighi. Here it is that the 

 black blood, brought by the pulmonary artery, changes its 

 color to that of red. Here commence the pulmonary veins; 

 and here in breathing animals begins the arterial circula- 

 tion. The pulmonary veins four in number, two for each 

 lung take the blood thus changed, and convey it to the 

 left ventricle of the heart, where ends the lesser or pul- 

 monary, and begins the greater or general circulation. The 

 Structure is the same as that of the arteries and veins already 

 described. 



There is one remarkable peculiarity in this circulation, 

 deserving notice. It is this : that its arteries carry venous 



