STRUCTURE OF ARTERIES. 



93 





blood and the fluids of the tissues. The fluid after it passes into the tissues consti- 

 tutes the lymph, and acts like a stream irrigating the tissue elements.] 



L The arteries are distinguished from veins by their thicker walls, due to the 

 greater development of smooth muscular and elastic tissues the middle coat 

 (tunica media) of the arteries is specially thick, while the outer coat (t. adventitia) 

 is relatively thin. [The absence of valves is by no means a characteristic feature.] 

 A typical artery consists of three coats (fig. 68). (1) The tunica intima, or inner coat, 

 consists of a layer of (a) irregular, long, fusiform nucleated squamous cells forming the 

 excessively thin transparent endothelium, immediately in contact with the blood-stream. 

 [Like other endothelial cells, these cells are held together by a cement substance, which is 

 blackened by the action of silver nitrate.] Outside this lies a very thin, more or less fibrous, 

 layer sub-epithelial layer in which numerous spindle or branched protoplasmic cells lie em- 

 bedded within a corresponding system of plasma canals. Outside this is an elastic lamina (b), 

 which in the smallest arteries is a structureless or fibrous elastic membrane in arteries of 

 medium size it is a fenestrated membrane (Henle), while in the largest arteries there may be 

 several layers of elastic laminae or fenestrated elastic membrane mixed with connective-tissue. 

 [In some arteries the elastic membrane is distinctly fibrous, the fibres being chiefly arranged 

 longitudinally. It can be stripped off, when it forms a brittle elastic membrane, which has a 

 great tendency to curl up at its margins. In a transverse section of a middle-sized artery it 

 appears as a bright wavy line, but the curves are probably produced by the partial collapse of the 

 vessel. It forms an important guide to the pathologist, in enabling him to determine which 

 coat of the artery is diseased. ] In middle-sized and large arteries a few non-striped muscular 

 fibres are disposed longitudinally between the elastic plates or laminae. Along with the circular 

 muscular fibres of the middle coat, they may act so as to narrow the artery, and they may also 

 aid in keeping the lumen of the vessel open and of uniform calibre. 



(2) The tunica media, or middle coat, contains much non-striped muscle (c), which in the 

 smallest arteries consists of transversely disposed non-striped muscular fibres lying between the 

 endothelium and the T. adventitia, while a finely granular tissue with few elastic fibres forms 

 the bond of union between them. As we proceed from the very smallest to the small arteries, 

 the number of muscular fibres becomes so great as to form a well-marked fibrous ring of non- 

 striped muscle, in which there is comparatively little connective-tissue. In the large arteries the 

 amount of connective-tissue is considerably increased, and between the layers of fine connective- 

 tissue numerous (as many as 50) thick, elastic fibrous or fenestrated laminae are concentrically 

 arranged. A few non-striped fibres lie 

 scattered amongst these, and some of them 

 are arranged transversely, while a few 

 have an oblique or longitudinal direction. 



The first part of the aorta and pulmonary 

 artery, and the retinal arteries, are devoid 

 of muscle. The descending aorta, common 

 iliac, and popliteal have longitudinal fibres 

 between the transverse ones. Longitudinal 

 bundles lying inside the media occur in 

 the renal, splenic, and internal spermatic 

 arteries. Longitudinal bundles occur both 

 on the outer and inner surfaces of the um- 

 bilical arteries, which are very muscular. 



(3) The tunica adventitia, or outer coat, 

 in the smallest arteries consists of a struc- 

 tureless membrane with a few connective- 

 tissue corpuscles attached to it ; in somewhat 

 larger arteries there is a layer of fine fibrous 

 elastic tissue mixed with bundles of fibrillar 

 connective-tissue (d). In arteries of middle 

 size, and in the largest arteries, the chief 

 mass consists of bundles of fibrillar con- ~ ... . _. ,_* ' , , , -, , 

 nective-tissue containing connective-tissue Capillaries. The outlines of the nucleated endo- 

 corpuscles. The bundles cross each other the . lial c r el \ s Wlth the ceraent blackened b y the 

 in a variety of directions, and fat cells often actlon of Sllver nitrate. 



lie between them. Next the media there are numerous fibrous or fenestrated elastic lamellae. 

 In medium sized and small arteries the elastic tissue next the media takes the form of an inde- 

 pendent elastic membrane (Henle's external elastic membrane). Bundles of non-striped muscle, 

 arranged longitudinally, occur in the adventitia of the arteries of the penis, and in the renal, 

 splenic, spermatic, iliac, hypogastric, and superior mesenteric arteries. 



II. The capillaries, while retaining their diameter, divide and. reunite so as to form net- 



