CHAP, iv.] THE VASCULAR MECHANISM. 187 



is a continual interchange of material between the blood in the 

 capillary, and the elements of the tissue outside the capillary, the 

 lymph acting as middle man. By this interchange the tissue 

 lives on the blood and the blood is affected by its passage through 

 the tissue. In the small arteries which end in, and in the small 

 veins which begin in the capillaries, a similar interchange takes 

 place ; but the amount of interchange diminishes as, passing in 

 each direction from the capillaries, the walls of the arteries and 

 veins become thicker ; and indeed, in all but the minute veins 

 and arteries the interchange is so small that it may practically 

 be neglected. It is in the capillaries (and minute arteries and 

 veins) that the business of the blood is done ; it is in these that 

 the interchange takes place ; and the object of the vascular 

 mechanism is to cause the blood to flow through these in a 

 manner best adapted for carrying on this interchange under 

 varying circumstances. The use of the arteries is in the main 

 simply to carry the blood in a suitable manner from the heart 

 to the capillaries, the use of the veins is in the main simply to 

 carry the blood from the capillaries back to the heart, and the use 

 of the heart is in the main simply to drive the blood in a suitable 

 manner through the arteries into the capillaries and from the 

 capillaries back along the veins to itself again. The structure 

 of these several parts is adapted to these several uses. 



The structure of arteries, capillaries and veins. 



105. On some features of connective tissue. The heart and 

 blood vessels are, broadly speaking, made up partly of muscular 

 tissue with its appropriate nervous elements, and partly of certain 

 varieties of the tissue known as connective tissue. We shall 

 have to speak of some of the features of connective tissue of phy- 

 siological importance when we come to deal with the lymphatic 

 system, for this system is intimately associated with connective 

 tissue. But an association only less close exists between the 

 blood vessels and connective tissue ; for connective tissue not only 

 enters largely, in one or other of its forms, into the structure of 

 the blood vessels, but also forms a sort of bed both for the larger 

 vessels on their way to and from the several tissues and organs and 

 for the smaller vessels, including the capillaries, within each tissue 

 and organ ; indeed a capillary may be regarded as a minute tubular 

 passage hollowed out in the connective tissue which binds together 

 the elements of a tissue. It will be desirable therefore to point 

 out at once a few of the characters of connective tissue. 



The connective tissue of the adult body is derived from certain 

 mesoblastic cells of the embryo and consists essentially of certain 

 cells, which do not lie in close contact with each other as do the 

 cells of epithelium, but are separated by more or less intercellular 

 material which may in certain cases be fluid or semi-fluid but 



