518 



THE ARTERIES. 



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

 possesses, nevertheless, some importance, as it greatly facilitates description 

 TERMINATION. The arteries terminate in 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 meshe* 



Fig. 266. 



Fig. 267. 



WEB OF FROG'S FOOT STRETCHING BETWEEN TWO TOES, SHOWING THE BLOOD- 

 VESSELS AND THEIR ANASTOMOSES. 

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



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

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

 The capillary system is, therefore, nothing more thai 

 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, withoul 

 passing through a capillary plexus. In describing the 

 genital organs we shall notice, in detail, the termination 

 of the arteries in the cavernous tissues. 



STRUCTURE. The walls of arteries offer a certain 

 rigidity, which permits these vessels to remain open 

 when they are emptied of blood. The ancients believed 

 EPITHELIAL CELLS OF this was their normal condition, and that they were 



BLOOD-VESSELS. 



' FronT^an barter ^ a P er ^ ect vacuum exists throughout the entire circulatory 

 Magnified 11 350 dja- system. The gaping of the arteries must be attributed 

 meters. solely to the physical properties of their walls. 



These wails comprise three superposed tunics : an 

 internal, middle, and external. 



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



