346 



THE CIKCULATION. 



Fig. 123. 



CAPILLARY PLEXUS, from the web of the 

 frog's foot. 



short intervals, in such a manner as to form an interlacing network or 

 plexus of minute bloodvessels, known as the capillary plexus. The 



vessels forming this plexus vary 

 somewhat in size, abundance, 

 and arrangement in different 

 parts of the body. In every 

 vascular organ and tissue there 

 are certain spaces or islets, in- 

 closed on all sides by capilla- 

 ries, but into the interior of 

 which these vessels do not pene- 

 trate. Such islets or intervas- 

 cular spaces must therefore ob- 

 tain their nourishment by the 

 exudation and absorption of the 

 fluid ingredients of the blood 

 through the capillary walls and 

 the substance of the intervening 

 tissue. 



The special arrangement of 



the capillary bloodvessels, and the form and size of the meshes of their 

 network, are, in general, characteristic of each separate organ or tissue. 

 In the muscles, the meshes are in the form of long parallelograms, cor- 

 responding with that of the muscular fibres ; in the mucous membranes 

 of the stomach and large intestine, they are hexagonal, or irregularly 

 circular, inclosing the orifices of the secreting follicles ; in the papillae 

 of the tongue and skin, and in the placental tufts, the capillaries form 

 twisted vascular loops ; in the glomeruli of the kidneys, convoluted 

 coils ; in the connective tissue, irregularly shaped figures, correspond- 

 ing in direction with the fibrous bundles of the tissue. 



The capillary bloodvessels are the most abundant, and interlaced in 

 the finest network, in those organs to which the blood is distributed for 

 other purposes than for local nutrition; as for that of aeration, secre- 

 tion, or absorption. One of the closest of all the capillary networks is 

 that of the lungs, in which the diameter of the spaces separating the 

 r bloodvessels, in the walls of the pulmonary vesicles, is sometimes a little 

 greater and sometimes a little less than that of the capillaries them- 

 selves. In the glandular tissue of the liver, the spaces separating the 

 adjacent vessels are only a little wider than the capillaries forming the 

 intra-lobular network. In the nerves, the serous membranes, and the 

 tendons, on the other hand, the capillary vessels are less closely inter- 

 woven ; and in the adipose tissue they form wide, open meshes, em- 

 bracing the exterior of the separate fat vesicles. 



Movement of the Blood in the Capillary Vessels. The motion of the 

 blood in the capillaries may be studied by examining, under the micro- 

 scope, any transparent tissue of a sufficient degree of vascularity. The 

 frog is the most convenient animal for this purpose, owing to the readi- 



