PHYSIOLOGY FOR BEGINNERS < H.M.. 



Many veins, especially those of the limbs, have valves placed 

 frequently along their course. A valve is formed by a semilunar 

 fold of the inner layer of the wall of the vein, which projects 

 into the vessel, and is directed towards the heart : it forms a 

 pocket like the pockets of the semilunar valves of the heart. 

 Sometimes there is only one such fold, sometimes there are 

 two or even three such folds at the same level. The blood 



FIG. 48. Diagram of the valves of veins. 



-I, a vein laid open to show pouch-shaped valves. C, capillary side ; and //, heart 

 side of a vein. The arrow showing the direction of the blood, in the upper figure 

 towards the heart, in the lower towards the capillaries. 



can pass freely through the valve towards the heart, but 

 any pressure on the vein above, or weight of blood in it, 

 only closes the valve so that the blood cannot return towards 

 the capillaries. 



The pulmonary vein and the inferior vena cava and the 

 superior vena cava have no valves, nor has the portal vein 

 itself, although the smaller veins of the portal system are well 

 supplied with them. No artery has valves ; the semilunar 

 valves at the beginning of the aorta and pulmonary artery 

 belong to the heart. 



Regulation of the Circulation 



Regulation of the Heart. We have seen that the 

 skeletal muscles are supplied with nerves, and that when a 

 muscle contracts, it is because an impulse from the spinal 

 cord or brain has reached it along its motor nerve. The heart 

 is also supplied with nerves, but there is this great difference, 

 namely, that the heart can go on beating without receiving 

 impulses along its nerves. When the heart of a frog is cut 

 out of the body it will go on beating for many hours if it 



