io8 SCIENCE PRIMERS, [§ vii. 



a continuation on a smaller scale of the same tube 

 which is seen in Fig. 15. This knot is covered by 

 a close network of capillaries, which at c are supposed 

 to be unravdled and taken away from the knotted 

 tube in order to show them. The capillaries, you 

 will understand, though inside the knot, are always out- 

 side the tube. If you were to drop a very diminutive 

 marble in at h (Fig. 15), it would rattle down the 

 corkscrew passage through the thick epidermis, shoot 

 down the straight tube b (Fig. 16), and roll through the 

 knot ^, until it came to rest at the blind end of the 

 tube. Along its whole course it would touch nothing 

 but cells, like the cells of the epidermis, a single layer 

 of which forms the walls of the tube where it runs 

 below the epidermis. If it got lodged at h (Fig. 1 5 ), or 

 got lodged in the knot at a (Fig. 16), it would in both 

 cases be touching epidermic cells. But there would 

 be this great difference. At h it would be ever so 

 far removed from any blood capillary ; at « it would 

 only have to make its way through a thin layer of 

 single cells, and it would be touching a capillary 

 directly. At h it might remain dry for some tune ; 

 at a it would get wet directly, for there is nothing to 

 prevent the fluid parts of the blood oozing out 

 through the thin wall of the capillaries, and so 

 through the thin wall of tlie tube into the canal of 

 the tube, on to the marble. 



In fact, the inside of the knot is always moist and 

 filled with fluid. When the capillaries round the 

 knot get over-full of blood, as they often do, a great 

 deal of colourless watery fluid passes from them into 

 the tube. The tube gets full, the fluid wells up right 

 into the corkscrew portion in the thickness of the 



