112 ANATOMY OF THE RABBIT 



action of the heart is muscular and rhythmic; contraction, or 

 systolic phases, alternating with expansion, or diastolic phases 

 (cf. p. 64). The flow is maintained in one direction principally by 

 the atrioventricular valves of the heart and by the semilunar valves 

 of the aorta and pulmonary arteries, though there are also valves in 



vc 



ap-^^ 



L-vVp 



9 



Fig. 63. Corrosion preparation of the blood-vessels of the lungs of a 

 rabbit, dorsal view: a, aorta; ap, pulmonary artery; vc, right superior vena 

 cava; vp, pulmonary vein. 



the course of some of the veins. The arteries are tubes with thicken- 

 ed elastic walls. They are expanded by the impulse of blood from 

 the heart, contraction of which is followed by a pulse wave in the 

 arteries. The passage of blood into the capillaries takes place more 

 slowly and uniformly, while the arteries contract to their previous 

 diameter and the relative distribution of blood to various parts is 



