26 A COURSE ON ZOOLOGY. 



is simultaneous contraction of both auricles, forcing the 

 blood into the ventricles, and consequent passive dilata- 

 tion of the ventricles by the inflowing liquid. At the 

 second interval there is contraction of the ventricles, and 

 forced passage of the blood into the aorta and pulmonary 

 artery, since the auriculo-ventricular valves prevent its 

 return into the auricles. The entrances to the arteries 

 are also provided with valves which prevent reflux of 

 blood into the ventricles when it has once entered the 

 vessels. 



That part of the circulatory course included between 

 the right ventricle and the left auricle is called the lesser 

 circulation, because of its lesser length, or the pulmo- 

 nary circulation, because it takes place in the lungs. The 

 pulmonary artery, which takes its origin from the right 

 ventricle, is very short ; a little over an inch from the 

 heart it bifurcates, and each branch passing to one lung 

 then divides into a great number of branches, which be- 

 come smaller and smaller. The last little threads are 

 distributed over the surface of the pulmonary vesicles, 

 and bring the liquid which they contain in almost direct 

 contact with the air. Other capillaries receive the blood, 

 which has now become red arterial blood, and form a 

 new system, which converges more and more, and at 

 length is composed of only four large vessels, which are 

 the pulmonary veins ; these veins empty into the left 

 auricle. It is well to observe that here the name artery 

 has been given to a vessel that carries venous blood, 

 while vessels that carry arterial blood are in this case 

 called veins. This naming is on account of the structure 

 of the vessels and not because of the character of the 

 blood they carry, and we must define arteries and veins, 

 not as vessels that carry, the ones arterial and the others 



