120 



STRUCTURE OF ARTERIES. 



intimate relation with the tissues. Through the excessively thin walls 

 of the capillaries the fluid part of the blood transudes, to nourish the 

 tissues outside the capillaries. [At the same time fluids pass from the 

 tissues into the blood. Thus, there is an exchange between the blood 

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

 constitutes the lymph, and acts like a stream irrigating the tissue 

 elements.] 



I. 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.] 



The arteries consist of three coats 

 (Fig. 40). (1.) The Tunica intima, 

 or inner coat, consists of a layer of 

 (a) irregular, long, fusiform nucleated 

 squamous cells forming the exces- 

 sively thin transparent endothelium 

 (His, 1866), 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 (&), which in the smallest 

 arteries is a structureless or fibrous 

 elastic membrane in arteries of medium 

 size it isafenestrated 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 may 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 



Fig. 40. 



Small artery to show the various 

 layers which compose its walls 

 a, endothelium; &, internal elastic 

 lamina ; c, circular muscular fibres 

 of the middle coat; d, the connective 

 tissue outer coat (T. adventitia). 



