x.] THE CIRCULATING SYSTEM. 409 



that of the pulmonary artery (so called because it conveys 

 blood to the lungs), which is guarded by three membranous 

 folds, termed the semilunar "valves. 



The left auricle has no less than four orifices ; namely, 

 those of the four pulmonary veins. Their openings are not 

 guarded by any valves. 



The left ventricle, much thicker than any other chamber 

 of the heart, has, besides the opening between it and the 

 left auricle, one large aperture guarded by three crescentic 

 flaps (the semilunar valves of the aorta), which valves re- 

 semble in structure those of the pulmonary artery. 



3. At its first embryonic appearance the heart is merely a 

 tube. In the DEVELOPMENT of the heart this tube becomes 

 bent upon itself and divided into two chambers. The pre- 

 axial chamber (nearest the head) becomes subdivided into 

 the ventricles ; the post-axial one (which alone receives blood 

 from the body) becomes the auricles which later grow to be 

 pre-axial to the ventricles. 



The further subdivision of each chamber into a right and 

 left segment is accomplished by the ingrowth of two septal 

 projections, which gradually become complete partitions, 

 though the partition between the auricles is not complete at 

 birth, an aperture existing then in that place which subse- 

 quently becomes the fossa ovalis. 



The septum between the ventricles arises in such a manner 

 as to divide the chamber giving origin to the pulmonary 

 artery from the one whence springs that great primary artery, 

 the aorta. 



4. Turning now to survey the heart as it exists in Verte- 

 brate animals generally, we find that different creatures exhibit 

 permanently conditions which are but transitory in man. 



In so far as the heart of man consists of four distinct cavities 

 it agrees with that of all other animals not only of his own 

 class, but also of the class of Birds together with the Croco- 

 dilian group. 



It may be that the ventricles are but imperfectly divided, 

 as is the case in all Reptiles except Crocodiles, in which 

 latter they are completely separated. 



The auricles themselves may also communicate perma- 

 nently, as in some Chelonians ; while, on the other hand, that 

 trace of primitive division, the fossa ovalis, may be more com- 

 pletely obliterated than in man, as is the case in the Kangaroo. 



Where (as in Batrachians) the heart has but a single ven- 

 tricle, the root of the aorta is dilated into a bulbus aort&, 



