LEPUS 147 



auricles and two ventricles. The truncus arteriosus has been 

 split into two, right down to its base. One of these vessels 

 opens out of the right ventricle and leads to the lungs ; it is 

 the pulmonary artery. The other opens out of the left ventricle 

 and is the aorta which leads to the carotid arteries and the 

 systemic arch. The two superior venae cavae and the inferior 

 vena cava open directly into the right auricle ; there is no sinus 

 venosus. The pulmonary veins open into the left auricle. 

 Guarding the opening between the right auricle and right 

 ventricle is the tricuspid valve ; the corresponding opening 

 between the left auricle and left ventricle is guarded by the 

 mitral valve.* The openings of the aorta and pulmonary 

 artery are guarded by semi-lunar valves. 



The systemic (4th arterial) arch persists only on the left 

 side. On the right, it is represented only by the short in- 

 nominate artery from which the right carotid and right sub- 

 clavian arteries arise. On the left side these two arteries arise 

 from the systemic arch, which, passing back and up round the 

 left side of the gut, becomes the dorsal aorta. The dorsal 

 aorta gives off the following arteries : coeliac (to stomach, 

 liver, duodenum, and spleen) ; anterior mesenteric (to small 

 intestine and colon) ; posterior mesenteric (to rectum) ; all of 

 which run ventrally in the mesentery to the several viscera. 

 Between the anterior and the posterior mesenteric arteries, 

 the dorsal aorta also gives off the renal arteries to the kidneys, 

 and the genital arteries to the gonads. In the case of males 

 in which the testes have descended into the scrotal sacs, 

 the latter arteries are of considerable length. Posteriorly the 

 dorsal aorta divides into the iliac arteries which supply the 

 hind legs, and the caudal artery. .The 5th arterial arch dis- 

 appears, but the 6th is represented by the pulmonary. 

 Originally, as in the fish, the 6th arterial arch communicated 

 with the lateral dorsal aorta, and this communication is present 

 in the mammalian embryo, on the left side, in the form of the 

 ductus arteriosus. In the adult the ductus arteriosus loses 

 its function (which is important in the embryo) and degenerates 



* The number of flaps which these valves possess should be obvious : 

 the tricuspid has three, and the mitral two. 



