THE VASCULAR SYSTEM. 



BY THE LATE ALFRED H. YOUNG, 



Professor of Anatomy, Victoria University, Manchester, 



AND 



ARTHUR EOBINSON, 



Professor of Anatomy, University of Birmingham. 



EEVISED BY ARTHUR EOBINSON, M.D., 



Professor of Anatomy in the University of Edinburgh. 



vascular system consists of a series of tubes, called vessels, which run 

 through all parts of the body. Some of the vessels contain a coloured fluid 

 called blood, others are filled with a colourless fluid, called lymph; hence the 

 distinction between the blood-vascular system and the lymph-vascular system. The 

 two systems differ, not only as regards their contents, but also in their relations 

 to the tissues amongst which they lie ; for whilst the vessels of the blood- vascular 

 system, with the possible exception of the splenic vessels, are closed, those of the 

 lymph-vascular system communicate with the serous sacs. 



The tubes or vessels of the blood-vascular system vary in size and in the 

 structure of their walls, but all contain blood, which is conveyed, through them, 

 to and from the tissue elements of the body. The blood is propelled along the 

 vessels chiefly by a central propulsive organ the heart. The outgoing vessels 

 from the heart, along which blood is transmitted to the tissues, are termed arteries ; 

 the vessels which return blood from the tissues to the heart are known as veins ; 

 whilst the smallest tubes those which connect the arteries and veins together, 

 constituting at once the terminations of the arteries and the commencements of 

 the veins are called capillaries. 



Blood capillaries are very small (hair-like) vessels, with exceedingly thin walls 

 which permit of the easy passage of the nutritive materials outwards from the 

 blood to the tissues, and, of the passage in the opposite direction, of some of the 

 products of tissue changes and of food material absorbed from the alimentary canal. 



Arteries and veins are simply conducting passages; structurally they differ 

 from capillaries in the greater complexity of their walls. They vary greatly in size, 

 but are always larger than capillaries. The calibres of the arteries and veins 

 increase progressively from the periphery up to the heart, where both sets of vessels 

 reach their greatest size. With the increase in calibre there is a corresponding 

 increase in the thickness and complexity of the walls of the vessels. 



Structure of Blood Capillaries. Blood capillaries measure from 8 p to 

 12-5 /A in diameter, and about *75 mm. in length. Their walls are simple, and, in 

 the smallest capillaries, consist of elongated elastic endothelial cells, with sinuous 

 edges, pointed extremities, and oval nuclei. The cells are cemented to one another, 

 along their margins, by intercellular cement, which readily stains with nitrate of 

 silver. Here and there the cement substance appears to accumulate, forming minute 

 spots indicative of the less perfect apposition of the edges of the cells. Such spots 

 when small, form the so-called stigmata ; when larger they are known as stomata. 



The larger capillaries are invested by a connective tissue sheath consisting of 

 branched cells which are united together and to the endothelial cells of the capillary 

 This sheath is termed the tunica externa capillaris. 



Capillaries are arranged in networks, the nature and character of which differ 



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