BLOOD CAPILLARIES 31 



but is in reality formed of a number of thin scales, called 

 "cells," cemented together at their edges (Fig. 7, A, e.c) ; 

 in each of these cells lies a small oval body (Fig. 7, n), 

 termed a nucleus. 



These tubes are the blood capillaries. They varv in 

 diameter from 14 ^ to 16 /x (^J^q to j^^^ of an iiich)i ; 

 they are sometimes disposed in loops, sometimes in long, 

 sometimes in wide, sometimes in narrow meshes ; and the 

 diameters of these meshes, or, in other words, the inter- 

 spaces between the capillaries, are sometimes hardly wider 

 than the diameter of a capillary, sometimes many times as 

 wide (see Figs. 25, 39, 58, and 64). These interspaces are 

 occupied by the substance of the tissue which the capillaries 

 permeate, so that the ultimate anatomical components of 

 every part of the body are, strictly speaking, outside the 

 vessels, or extm-vasadar. 



But there are certain parts of the body in which these 

 blood-capillaries are absent. These are the epidermis 

 and epithelium, the nails and hairs, the substance of the 

 teeth, and to a certain extent the cartilages and the trans- 

 parent coat (cornea) of the eye in front ; which may 

 and do attain a very considerable thickness or length, and 

 yet contain no blood-vessels. However, since we have seen 

 that all the tissues are really extra-va.scular, these differ 

 only in degree from the rest. The circumstance that all 

 the tissues are outside the vessels by no means interferes 

 with their being bathed by the fluid which is inside the 

 vessels. In fact, the walls of the capillaries are so ex- 

 ceedingly thin that their fluid contents readily exude 

 through the delicate membrane of which they are com- 

 posed, and irrigate the tissues in which they lie. 



2. The Arteries and Veins. — The capillary tubes so 

 far described contain, during life, the red fluid, blood, 

 and are continued on opposite sides, into somewhat larger 



1 The ordinarily used unit of histological nieas\irement is fn\,g of a 

 millimeter, and is usually represented by the Greek letter fi, which 

 sta:ids for micro-millimeter. Since one millimeter is very nearly ^ of 

 an inch iJi = ^,s„ of an inch. 



