CHAPTER VIII. 



THE VASCULAR SYSTEM: THE BLOOD. 



Having studied the four distinctive tissues of the body (the 

 epithelial, connective, muscular, and nervous), their structure, 

 position in the body, and the various functions they are espe- 

 cially adapted to perform, we shall next consider the vascular, 

 respiratory, alimentary, and excretory systems, by means of 

 which all the tissues are supplied with the materials necessary 

 for their life and growth, and relieved of all those waste and 

 superfluous matters which are the results of their activity. 



All the tissues of the body are traversed by minute tubes, 

 called capillary blood-vessels, to which blood is brought by 

 large tubes, called arteries, and from which blood is carried 

 away by other large tubes, called veins. These capillaries form 

 networks, the meshes of which differ in form and size in the 

 different tissues. The meshes of these networks are occupied 

 by the elements (cells or their products) of the tissues ; and 

 filling up such spaces as exist between the capillary walls and 

 the elements of the tissue, is found a colourless fluid, resembling 

 in many respects the fluid portion of the blood, and called 

 lymph. As the blood flows through the capillaries, certain 

 constituents of the blood pass through the capillary wall into 

 the lymph, and certain constituents of the lymph pass through 

 the capillary wall into the blood within the capillary. There 

 is thus an interchange of material between the blood within the 

 capillary and the lymph outside. A similar interchange of 

 material is at the same time going on between the lymph and 

 the tissue itself. Hence, by means of the lymph acting as 

 middleman, a double interchange of material takes place 

 between the blood within the capillary and the tissue outside 

 the capillary. In every tissue, so long as life lasts and the 



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