70 A MANUAL OF PHYSIOLOGY 



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 made up of short oblong cells, devoid of 

 a sarcolemma, often branched, and arranged in anastomosing 

 rows. Each cell has a single nucleus in the middle of it. 

 The fibres are transversely striated, but the striae are not so 

 distinct as in skeletal muscle. Many fibres pass from one 

 auricle to the other, and from one ventricle to the other. 

 The auricles and ventricles are also, in some mammals at 

 least, connected in early life by muscular tissue ; and even in 

 the adult traces of this connection may persist (Plate I., 4). 



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. 



The problems of the circulation are partly physical, partly 

 vital. Some of the phenomena observed in the blood-stream 

 of a living animal can be reproduced on an artificial model ; 

 and they may justly be called the physical phenomena of the 

 circulation. Others are essentially bound up with the pro- 

 perties of living tissues ; and these may be classified as the 

 vital or physiological phenomena of the circulation. The 



