74 A MANUAL OF PHYSIOLOGY 



thelial cells. In the capillaries nothing else is present ; the endo- 

 thelial 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, j valves are present in many veins. 

 These are semilunar folds of thb internal coat projecting into the 

 lumen in such a direction as toj 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 ojf 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 

 differentiated. 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 continuous sheet of tissue anastomosing in every direc- 

 tion. 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 continuous sheet of muscular tissue. 



