CHAPTER IV. 



THE VASCULAR MECHANISM. 



SEC. 1. THE STRUCTURE AND MAIN FEATURES OF 

 THE VASCULAR APPARATUS. 



93. THE blood, as we have said, is the internal medium on 

 which the tissues live ; from it these draw their food and oxygen, to 

 it they give up the products or waste matters which they form. The 

 tissues, with some few exceptions, are traversed by, and thus the 

 elements of the tissues surrounded by, networks of minute, thin- 

 walled tubes, the capillary blood vessels. The elementary striated 

 muscle fibre, for instance, is surrounded by capillaries, running in 

 the connective tissue outside but close to the sarcolemma, arranged 

 in a network with more or less rectangular meshes. These capil- 

 laries are closed tubes with continuous walls, and the blood, which, 

 as we shall see, is continually streaming through them, is as a 

 whole confined to their channels, and does not escape from them. 

 The elements of the tissues lie outside the capillaries, and form 

 extra-vascular islets of different form and size in the different 

 tissues, surrounded by capillary networks. But the walls of the 

 capillaries are so thin and of such a nature that certain of the 

 constituents of the blood pass from the interior of the capillary 

 through the capillary wall to the elements of the tissue outside 

 the capillary, and, similarly, certain of the constituents of the 

 tissue, to wit, certain substances, the result of the metabolism 

 continually going on in the tissue, pass from the tissue outside 

 the capillary through the capillary wall into the blood flowing 

 through the capillary. Thus, as we have already said, 13, there 



