380 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



bright blood as arterial and the dark blood as venous. 

 This is true only of the systemic circulation. The very 

 reverse is true of the pulmonic circulation. 



Structure of the Heart : The Valves.— We have 

 given the course of the circulation ; but the question 

 arises, How is it maintained continuously in the same 

 direction ? This is done in the same way as it is done 

 in machinery — viz., by a system of valves which prevents 

 its going in the other direction. 



The valves of the heart are of two kinds, which may 

 be called curtain valves and pocket valves (semilunar 

 valves). The former are between the two chambers of 



one heart, to prevent 

 the blood from going 

 back to the auricle; 

 the latter are placed 

 at the outlet of the 

 ventricles into the 

 great outgoing arter- 

 ies, i. e., at the base 

 of the aorta and the 

 pulmonary artery to 

 prevent the blood dis- 

 charged into these ar- 

 teries from falling 

 back into the heart 

 when the ventricle ex- 

 pands again. The cur- 

 tain valves I so call because they are thin membranous 

 curtains between the two chambers of the heart and 

 opening always into the ventricle. From the interior of 

 the wall of the ventricle there go cords (heart strings), 

 which are attached to the edges of the curtain, so that 

 while these open easily into the ventricle, allowing blood 

 to pass (Fig. 261, A), yet if blood attempts to pass back 



Fig. 261. — Left heart. A, auricle contracted, 

 ventricle expanded. B, ventricle con- 

 tracted, auricle expanded : pv, pulmo- 

 nary vein ; au, auricle ; w, ventricle ; ao t 

 aorta. 



