CHAPTER VII. 



THE VASCULAR SYSTEM: THE BLOOD. 



HAVING studied three of the distinctive tissues of the body 

 (the epithelial, connective, and muscular), their structure, posi- 

 tion in the body, and the various functions they are especially 

 adapted to perform, we shall next consider the vascular, respi- 

 ratory, 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. 1 



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 fibres) of the tissues ; and filling up 

 such spaces as exist between the capillary walls and the ele- 

 ments 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. 



1 As the nervous tissue is the most highly organized of the tissues, and its 

 functions are very complex and difficult to understand, the consideration of it is 

 deferred until the student has mastered the vascular, respiratory, alimentary, 

 and excretory systems. 



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