576 THE VASCULAR SYSTEMS 



artery above anastomosing with branches from the vessels below ; these anastomoses 

 are of considerable interest to the surgeon, as it is by their enlargement that a 

 collateral circulation is established after the application of a ligature to an artery. 

 The smaller branches of arteries anastomose more frequently than the larger, 

 and between the smallest twigs these inosculations become so numerous as to 

 constitute a close network that pervades nearly every tissue of the body. A ter- 

 minal artery is one which forms no anastomoses. Such vessels are believed to exist 

 in the brain, spleen, kidneys, lungs, mesentery, and papillary layer of the skin. 



Throughout the body generally the larger arterial branches usually pursue a 

 straight course, but in certain situations they are tortuous; thus, the facial arteries 

 in their course over the face, and the arteries of the lips, are extremely tortuous in 

 their course, to accommodate themselves to the movements of the parts. The 

 uterine arteries are also tortuous, to accommodate themselves to the increase of 

 size which the organ undergoes during pregnancy. 



The arteries are dense in structure, of considerable strength, highly elastic, and,, 

 when divided, they usually preserve, although empty, their cylindrical form. 



Histology of Arteries and Capillaries. An artery consists of an endothelial tube covered 

 by certain accessory coats. 



The coats of an artery are: (1) internal coat, or tunica intima; (2) a middle coat, or tunica 

 media; and (3) an external coat, or tunica adventitia (Fig. 443).y^,^f 



1. The inner coat (tunica intima} consists of endothelial cells resting upon some subendo- 

 thelial fibroelastic tissue. Limiting the intima is a wavy band of yellow elastic tissue called 

 the internal elastic lamina. In small arteries the endothelial cells rest upon the elastic lamina. 

 In large arteries (aorta, pulmonary a.) the elastic tissue forms the fenestrated membrane of 

 Henle. 



2. The middle coat (tunica media} consists of muscle, elastic tissue, and white fibrous tissue. 

 In medium-sized arteries the smooth muscle tissue is circularly arranged, with only a small 

 quantity of elastic tissue here and there. In small arteries the elastic tissue is absent; in the 



large arteries the elastic tissue predomi- 

 nates; in some vessels (retinal, first part 

 of aorta, and pulmonary artery) the 

 elastic tissue may entirely replace the 

 muscle tissue. Occasionally longitudi- 

 nally arranged muscle tissue is seen in 

 the media. In medium-sized arteries the 

 media is bounded by a layer of elastic 

 tissue called the external elastic lamina. 



-^^^^^r~'-:' r ^^S : ^~^ 3. The external coat (tunica adventitia) 



is called the fibrous coat. It contains 

 FIG. 428. Transverse section of part of the wall of the fibroelastic tissues, and in some arteries 



posterior tibial artery. X 75. A. Endothelial and sub- fibres of nonstriated muscle longitudi- 

 trated membrane") of faner coat, 'appearing a a^right^ne nally arranged. It contains the nutrient 

 in section. C. Muscle layer (middle coat). D. Outer coat, vessels, nerves, and Ivmphatics of the 

 ySbt&fStfS?!^^ arteries. As the arteries, diminish in 



cially near the muscular coat, a number of elastic fibres cut s j ze t ne coats likewise become thinner. 



The endothelial cells rest upon the in- 

 ternal elastic lamina; the media becomes 



reduced to a few layers of muscle fibres, and the adventitia is represented by some bundles of 

 fibroelastic tissue. This represents the precapillary arteriole, and it gradually becomes the 

 capillary. 



Capillaries are small endothelial tubes connecting arterial and venous systems. They vary 

 from -S^OTT to ?oV(7 of an inch (5 n to 13 p) in diameter, and about JTT of an inch (500 p} in 

 length. The endothelial cells are thin, flat, and irregular in outline; the darkly staining nucleus 

 usually causes a bulging of the cell, as it is thicker than the protoplasmic portion of the cells. 

 These cells are held together by a small amount of cement substance, and are considered by 

 many to have the property of phagocytosis. Small openings called stomata are frequently noted 

 between these cells, but they are considered artefacts. Capillaries anastomose and form vast 

 networks. Ampullae, sinusoids, retia mirabilia, sinuses, and anastomoses are forms of capillaries 

 seen in certain organs and tissues. 



Bloodvessels of the Bloodvessel Wall. Many small bloodvessels, the vasa vasorum, 

 enter the adventitia; from these vessels branches are sent into the media, but not the intima. 



