THE BLOODVESSELS. 49 



Each auricle communicates only with its corresponding 

 ventricle (Figs. 31 and 34). 



The two ventricles occupy the lower part of the heart, and 

 do not communicate with each other, but with their cor- 

 responding auricles. The orifices are called the auriculo- 

 ventricular orifices, right and left. The left cavities of the 

 heart contain arterial blood ; the right, venous. The auricles 

 having to propel the blood only into the ventricles, are not so 

 fleshy in their walls as the ventricles; and of these the left is 

 much the stronger, as it has to drive the blood through the 

 whole body excepting the lungs ; whilst the right ventricle 

 merely propels it through the lungs. 



Vena Cava Sup. Art.Pulm. Aorta. Art.Pulm. 



Pulmonary Veins. ''-'-- 

 Eight Auricle. 



Vena Cava Inferior. -"" 



Septum. Aorta. 

 Fig. 31. Theoretical Section of the Heajrt in Man. 



93. The Bloodvessels. These vessels are divided into 

 arteries and veins. The walls of these tubes are formed of 

 membranes or tissues. In the arteries there are three tissues : 

 the inner, continuous with the inner membrane of the heart, 

 resembles the serous membranes; the middle tissue is 

 fibrous and elastic ; the outer tissue, cellular and also elastic. 

 The fibres of the middle tissue are disposed circularly. In 

 the veins, the middle tissue is nx>t so distinct, being composed 

 merely of fibres, irregularly disposed ; these are soft, exten- 

 sible, and longitudinal. Thus the physical properties of the 

 veins and arteries differ widely. The veins have thin walls, 

 which collapse when the vessels are empty ; and they heal 



