THE CIRCULATION OF THE BLOOD. 259 



which, by contracting, force it out, are called ventricles. 

 The walls of the heart are composed of strong muscular or 

 fleshy fibres, crossing and interlacing one another, and those 

 of the ventricles are much thicker than those of the auri- 

 cles. 



9.(jThe cavities in the right side of the heart are trian- 

 gular in shape, and those of the left, oval. Each cavity 

 will hold about two ounces of blood. } Between each of 

 these two cavities or chambers, there is a passage, which is 

 closed with a valve ; so that when the ventricles contract, 

 the blood, instead of passing through into the auricle, is 

 poured into the arteries^ /This valve is formed by the dou- 

 bling of a thin, transparent membrane, which lines the 

 cavities, and extends both through the veins and arteries. 

 The valve in the right side of the heart is called tricuspid, 

 and that in the left, bicuspid or mitred) ' V 



10. There are also valves placed at the orifice of the 

 great arteries of the heart, the pulmonary and aorta, as they 

 are given off from the right and left ventricles. ^They differ 

 somewhat from the former, by being of a half-moon or semi- 

 lunar shape, and formed by folds of the lining membrane of 

 the arteries.) These are called sigmoid valves ; and their 

 use is f;o prevent the blood from flowing back from the arte- 

 ries into the ventricles) Where the vena cava, or the great 

 vein which returns all the blood, empties into the right 

 auricle of the heart, there is another valve, called the Eusta- 

 chian valve. (See Fig. 2.) 



11. (The arteries are the vessels into which the blood is 

 immediately propelled by the action of the heart, and which 

 distribute it to all the parts of the bodvl The pulmonary 

 artery or the artery of the right side of the heart, circulates 

 dark-coloured or venous blood through the lungs, where by 

 exposure to the air, it becomes a bright scarlet. This is re- 

 turned into the left auricle, which forces it into the left ven- 

 tricle, whence it is sent through the aorta to every part of 

 the system. 



