82 THE CIRCULATION OF THE BLOOD AND LYMPH 



The whole vascular system is lined with a single layer of endothelial 

 cells. In the capillaries nothing else is present; the endothelial layer 

 forms the whole thickness of the wall. In young animals, at any rate, 

 the endothelial cells of the capillaries are capable of contracting when 

 stimulated ; and changes in the calibre of these vessels can be brought 

 about in this way. |The walls of the arteries and veins are chiefly 

 made up of two kinds of tissue, which render them distensible and 

 elastic: non-striped muscular fibres and yellow elastic fibres. The 

 muscular fibres are mainly arranged as a circular middle coat, which, 

 especially in the smaller arteries, is relatively thick. One conspicuous 

 layer of elastic fibres marks the boundary between the middle and 

 inner coats. In the larger arteries elastic laminae are also scattered 

 freely among the muscular fibres of the middle coat. The outer coat 

 is composed chiefly of ordinary connective tissue. The veins differ 

 from the arteries in having thinner walls, with the layers less distinctly 

 marked, and containing a smaller proportion of non-striped muscle 

 and elastic tissue ; although in some veins, those of the pregnant uterus, 

 for instance, and the cardiac ends of the large thoracic veins, there is 

 a greater development of muscular tissue. Further, and this is of prime 

 physiological importance, valves are present in many veins. These 

 are semilunar folds of the internal coat projecting into the lumen in 

 Nsuch a direction as to favour the flow of blood towards the heart, 

 but to check its return. In some veins, as the venae cavae, the pulmonary 

 veins, the veins of most internal organs, and of bone, there are no valves; 

 in the portal system they are rudimentary in man and the great majority 

 of mammals. The valves are especially well marked in the lower limbs, 

 where the venous circulation is uphill. When a valve ceases to perform 

 its function of supporting the column of blood between it and the 

 valve next above, the foundation of varicose veins is laid; the valve 

 immediately below the incompetent one, having to bear up too great 

 a weight of blood, tends to yield in its turn, and so the condition spreads. 

 The smallest veins, or venules, are very like the smallest arteries, or 

 arterioles, but somewhat wider and less muscular. The transition 

 from the capillaries to the arterioles and venules is not abrupt, but 

 may be considered as marked by the appearance of the non-striped 

 muscular fibres, at first scattered singly, but gradually becoming closer 

 and more numerous as we pass away from the capillaries, until at length 

 they form a complete layer. 



In the heart the muscular element is greatly developed and differ- 

 entiated. Both histologically and physiologically the fibres seem to 

 stand between the striated skeletal muscle and the smooth muscle. In 

 the mammal the cardiac muscular fibres are generally described as 

 made up of short oblong cells, devoid of a sarcolemma, often branched, 

 and arranged in anastomosing rows, each cell having a single nucleus 

 in the middle of it. But it has recently been shown that the muscle 

 fibrils run right through the apparent cell boundaries, and form a con- 

 tinuous sheet of tissue anastomosing in every direction. The fibres 

 are transversely striated, but the striae are not so distinct as in skeletal 

 muscle. A sarcolemma is not absent, although it is more delicate 

 than in skeletal muscle, and perhaps of a different nature. Many 

 fibres pass from one auricle to the other, and from one ventricle to the 

 other. 



In the frog's heart the muscular fibres are spindle-shaped, like those 

 of smooth muscle, but transversely striated, like those of skeletal 

 muscle. From the sinus to the apex of the ventricle there is a con- 

 tinuous sheet of muscular tissue. 



