THE CIRCULATORY SYSTEM. 109 



laries, and the smaller veins, a part of the plasma of the blood, 

 somewhat modified in composition, makes its way through the vas- 

 cular walls, partly by osmosis, partly by a sort of filtration, and 

 becomes the nutrient lymph of the tissues. The composition of 

 this lymph varies a little in the different parts of the body, and 

 this variation is attributed to some kind of activity, allied to secre- 

 tion, on the part of the cells lining the vessels. 



The larger veins are provided with pocket-like valves, which 

 collapse when the blood-current is toward the heart, but which fill 

 and occlude the veins when, for any reason, the current is reversed. 

 When, therefore, the muscles contiguous to the larger veins thicken 

 during contraction and press upon the veins the effect is to urge 

 the blood within them in the direction of the heart. This accessory 

 mode of propulsion materially aids the heart, especially during 

 active exercise, when the muscles are in need of an abundant supply 

 of oxygen. 



The large lymphatic vessels are similarly provided with valves, 

 and valves guard the orifices by which the lymphatic trunks open 

 into the veins. But the chief reason for the flow of the lymph 

 appears to be the continuous formation of fresh lymph, which 

 drives the older fluid before it the so-called vis a tergo. 



For convenient description we may divide the vascular organs 

 into the heart, arteries, veins, capillaries, and lymphatics. 



1. The heart is covered externally by a nearly complete invest- 

 ment of serous membrane, the epicardium, which is a part of the 

 wall of the pericardial serous cavity. Its free surface is covered 

 with a layer of endothelium resting upon areolar fibrous tissue, and 

 containing a variable amount of fat. 



The substance of the heart is made up of a series of interlacing 

 and connected layers of cardiac muscular tissue, separated by layers 

 of areolar tissue, which extends into the meshes of the muscle, form- 

 ing the interstitial tissue of the heart. The fibres in the different 

 layers of muscle run in different directions, so that sections of the 

 wall of the heart show the individual muscle-cells cut in various 

 ways. 



The areolar tissue is more abundant and denser near the orifices 

 of the heart, and at the bases of the valves merges into dense 

 fibrous rings, which send extensions into the curtains of the valves, 

 increasing their strength and giving them a firm connection with 

 the substance of the organ. In the centre of the heart, between 



