308 AN INTRODUCTION TO ZOOLOGY 



The centre of the system is the heart, and while the heart of the 

 rabbit is typically mammalian, yet owing to its small size it is not 

 easy to examine, and after considering the rest of the circulatory 

 system we shall return to consider a mammalian heart as exemplified 

 in a larger animal such as the pig or sheep. It is sufficient to note 

 only the main points here. The heart consists of four chambers, 

 two atria with auricular appendages and two ventricles, a right and 

 a left in each case, and the cavities of one side of the heart have no 

 means of communicating directly with those of the other. Into 

 these chambers open or from them come off the main vessels, there 

 being no structures corresponding with the sinus venosus or conus 

 arteriosus of Rana. 



Arterial System. 



The arterial system is divided into two portions, the 

 pulmonary and the systemic. The pulmonary artery takes origin 

 from the antero-median corner of the right ventricle, passes forwards 

 and arches over to the dorsal side of the left atrium where it divides 

 into two branches, one going to each lung. The base of the pulmonary 

 artery is guarded by three tough membranous pocket-shaped 

 semilunar valves. 



The systemic arteries are all served by a single large trunk, the 

 aorta or aortic arch, which arises from the left ventricle and, passing 

 forwards dorsal to the pulmonary artery arches, over the atrium, 

 and the left bronchus reaches the left side of the vertebral column 

 along which it runs backwards as the dorsal aorta. Upon entering 

 the abdomen it takes up a median position below the centra. Just 

 before the pulmonary artery divides into two it is joined to the 

 aorta by a band of fibrous tissue, the ligamentum arteriosum, which 

 in the embryo is an open tube, the ductus arteriosus, and is the 

 remnant of the connection between these vessels in the embryo 

 and also in the primitive vertebrate, just as the ligamentum caroticum 

 of Rana is the persistent remains of another of these connections. 

 The base of the aorta is guarded by a series of three pocket-shaped 

 semilunar valves, similar to those in the pulmonary artery, which 

 prevent the blood passing back again into the ventricle. Be- 

 hind two of these valves are the openings of two small coronary 

 arteries which carry blood to the tissues of the heart itself. Just 

 at the point where the aorta is arching over it gives rise to a short 

 stout innominate artery, turning off to the right side, which after 

 a short course divides into the right common carotid and right sub- 

 clavian arteries. The right common carotid artery runs forward 

 alongside the trachea, giving off branches to it and other tissues 

 of the neck up to the level of the angle of the jaw. Here it divides 



