HEART 371 



across the hollow of one of the second row of semilunar valves. 

 It is from the outer side of this valve that the spiral valve starts. 

 Thus it comes about that the outer ends of the two parts of the 

 conus are each guarded by one and a half valves. The dorsal 

 chamber of the aorta communicates behind with the pulmocu- 

 taneous part of the conus and in front with the blood vessel to the 

 lungs (pulmocutaneous arch) ; the ventral chamber communicates 

 behind with the aortic part and in front with the blood vessels 

 known as the systemic and carotid arches. 



HEART-BEAT 



The function of the heart is, by the contractions of its muscular 

 wall which are known as its beat, to drive blood through the 

 vascular system to all parts of the body. The contraction starts 

 in the sinus venosus, driving the contained blood into the right 

 auricle. Meanwhile the left auricle is filling by the inflow of blood 

 from the lungs through the pulmonary vein. The auricles now 

 contract simultaneously, driving the blood into the ventricle. 

 The sinus is beginning to relax, but the reflux of blood into it is 

 prevented by the sinu-auricular valves. The right-hand side of 

 the ventricle receives the blood from the right auricle and the 

 left-hand side that from the left auricle, but these portions of 

 blood mix rapidly so that there is not, as is generally imagined, 

 any further separation of the two streams. The ventricle contracts 

 immediately after the auricles, the auriculo- ventricular valves 

 preventing the passage of blood back into the latter. The effect 

 of the contraction of the ventricle is therefore to drive the blood 

 onward into the truncus arteriosus. Both parts of this are filled, 

 and the blood flows into the carotid, systemic, and pulmo- 

 cutaneous arches nearly, if not quite, simultaneously. The heart 

 of the frog, with its single ventricle, is perhaps not primitive, and 

 the muscles of the wall seem to get most of their oxygen from the 

 blood in the chambers, since the single coronary artery is small. 



CIRCULATION OF THE BLOOD 



To and from the heart leads a complicated system of blood 

 vessels, through which the red blood is driven by the heart-beat. 

 The vessels which lead from the heart are called arteries ; those 

 which lead to the heart are veins. The arteries have thick, muscular 



