54 MANUAL OF ELEMENTARY ZOOLOGY 



chamber. 1 The dorsal chamber communicates behind with 

 the cavum pulmocutaneum and in front with the blood 

 vessel to the lungs (pulmocutaneous arch) ; the ventral 

 chamber communicates behind with the cavum aorticum 

 and in front with the blood vessels known as the systemic 

 and carotid arches. 



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 

 Heart-beat 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, and 

 these portions of blood mix slowly because a great part of 

 the hollow of the ventrical is spongy, owing to the presence 

 of muscular projections known as columna carnece. 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. Since this is on the right side of the 

 ventricle, it will receive first the blood from the right 

 ventricle. Both cavum aorticum and cavum pulmocutaneum 

 are filled, but since the pressure in the carotid and systemic 

 arches is higher than that in the pulmocutaneous arch the 

 blood is driven into the latter. As the ventricle continues 

 to contract the pressure of the blood rises until it is high 

 enough to overcome the resistance in the systemic and 

 finally in the carotid arches. At the same time the con- 

 traction of the truncus arteriosus brings the spiral valve into 

 a position in which it shuts off the cavum pulmocutaneum. 



1 The septum which makes this division ends towards the heart by 

 cutting 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 cavum aorticum and 

 cavum pulmocutaneum are each guarded by one and a half valves. 



