THE BLOODVESSELS. 55 



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

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

 whole hody excepting the lungs ; whilst the right ventricle 

 merely propels it through the lungs. 



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 not 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 

 easily when wounded. On the contrary, the arteries when 

 cut across, remain open, and when wounded do not heal so 

 perfectly ; in order to close, they must be obliterated, either 

 by pressure or by the use of the ligature. 



94. Arterial System. From a single artery, called the 

 aorta, springing from the left ventricle of the heart, all the 

 arteries of the body arise. It must, however, be borne in 

 mind that the artery called the pulmonary springs from the 

 right ventricle, and carries the venous blood to the lungs. 

 The accompaning figure (41) explains perfectly the course of 

 this great artery (the aorta), from its commencement in the 

 heart to its termination ; also of all the great branches 

 which arise from it. 



95. Venous System. The veins originating in the 

 capillary vessels, in which the arteries terminate, follow . 

 pretty nearly the course of the arteries ; but they are more 

 numerous and more superficial. Many are situated imme- 

 diately beneath the skin, imbedded in the superficial fascia of 

 the body. Others follow the course of the arteries, to termi- 

 nate however at length in two large trunks, which empty 

 themselves into the right auricle of the heart (Fig. 39). ^ 



The veins of the intestines present this remarkable anomaly 

 that they unite into a single trunk, which, instead of joining 

 the venous system, directly proceeds towards the liver, and is 

 ramified through that organ after the manner of an artery ; 



* A smaller vein, called coronary, returns the blood which has circulated 

 in the walls of the heart into the same cavity. R. K. 



