518 



THE ARTERIES. 



is not always easy to establish, and is far from having an absolute value; it 

 possesses, nevertheless, some importance, as it greatly iacilitates description. 

 Termination.— The arteries terminate m the substance of the tissues 

 by extremely fine and numerous ramuscules, which so frequently anastomose 

 with each other as to form a plexus or microscopical network, whose meshes 



Fig. 266. 



WEE OF FROG S FOOT STRETOHING BETWEEN TWO TOES, SHOWINO THE BLOOD- 

 VESSELS AND THEIR ANASTOMOSES. 



a, a, Veins ; 6, 6, 6, Arteries, the capillaries being between. 



Fig. 267. 



are very close. These ramuscules constitute the capillary system, which 

 again gives rise to ramifications of gradually increasing size, the veins. 

 The capillary system is, therefore, nothing more than 

 a network of microscopical canals intermediate to the 

 arteries and veins. 



In the erectile tissues, the mode of termination is 

 different : the small arteries sometimes opening directly 

 into the cells placed at the origin of the veins, without 

 passing through a capillary plexus. In describing the 

 genital organs we shall notice, in detail, the termination 

 of the arteries in the cavernous tissues. 



Stkuctubb. — The walls of arteries offer a certain 

 rigidity, which permits these vessels to remain open 

 when they are emptied of blood. The ancients believed 

 this was their normal condition, and that they were 

 filled with air during life. This was a grave error, as 

 a perfect vacuum exists throughout the entire circulatory 

 system. The gaping of the arteries must be attributed 

 solely to the physical properties of their walls. 



These walls comprise three superposed tunics : an 

 internal, middle, and external. 



The internal tunic is continuous with the endocardium of the left heart 



EPITHELIAL CELLS OF 

 BLOOD-VESSELS. 



ii, b. From a rein ; c. 

 From an artery, 

 Magnified 350 dia- 

 meters. 



