280 CIRCULATION. 



The true capillary vessels present the following charac- 

 teristics : 



1. Simplicity of Structure. They have but the single 

 amorphous coat, from -g-yj-g^ to -^TOT of an inch thick, the 

 continuation of the lining membrane of the larger vessels ; 

 not provided with an epithelial lining, but presenting, im- 

 bedded in its thickness, a number of oval nuclei with their 

 long diameters in the direction of the axis of the vessel. 



2. Small Diameter of the Vessels. Their diameter is gen- 

 erally as small or smaller than that of the blood-corpuscles ; 

 so that these bodies always move in a single line, and must 

 become deformed in passing through the smallest vessels; 

 recovering their natural shape, however, when they pass into 

 vessels of larger size. The capillaries are smallest in the 

 nervous and muscular tissue, retina, and patches of Peyer, 

 where they have a diameter of from -^^ to ^innr f an inch. 

 In the mucous layer of the skin, and in the mucous mem- 

 branes, they are from T Vo to -^Vo f an mcu ^ n diameter. 

 They are largest in the glands and bones, where they are from 

 ^A^ to -2-gVjr f an mc h m diameter. 1 . These measurements 

 indicate the size of the vessel, and not its caliber. Taking 

 out the thickness of their walls, it is only the very largest of 

 them which will admit of the passage of a blood-disk without 

 a change in its form. 



3. Peculiarities of Distribution. Unlike the arteries, 

 which grow smaller as they branch, and simply carry blood 

 by the shortest course to the parts, and the veins, which be- 

 come larger as we follow the course of the blood by union 

 with each other, the capillaries form a true plexus of vessels 

 of nearly uniform diameter, branching and inosculating in 



capillaries into three varieties, the first with a homogeneous coat, the second with 

 the addition of the muscular coat, and the third with the muscular and fibrous 

 coat, was made by Henle, and is, perhaps, the one most generally adopted. Kol- 

 liker gives the division we have adopted, regarding as true capillaries only those 

 vessels which have a single coat. The others he calls "vessels of transition." 

 1 KOLLIKER, Manual of Human Microscopic Anatomy, London, 1860, p. 500. 



