86 



CIRCULATION OF THE BLOOD. 



In vessels of considerable size, as well as in some capillaries, the corpuscles, occupy- 

 ing the central portion, move with much greater rapidity than the rest of the blood, 

 leaving a layer of clear plasma at the sides, which is nearly motionless. This curious 

 phenomenon is in obedience to a physical law regulating the passage of liquids through 

 capillary tubes for which they have an attraction, such as exists, for example, between 

 the blood and the vessels. In tubes reduced to a diameter approximating that of the 

 capillaries, the attractive force exerted by their walls upon a liquid, causing it to enter 

 the tube to a certain distance, called capillary attraction, becomes an obstacle to the pas- 

 sage of fluid in obedience to pressure. Of course, as the diameter of the tube is re- 



FIG. 31. Small artery and capillaries from tJie lung of a frog ; magnified 500 diameters. (From a photograph 

 taken at' the United States Army Medical Museum.) 



duced, this force becomes relatively increased, for a larger proportion of the liquid con- 

 tents is brought in contact with it. When we come to the smallest arteries and veins, 

 and still more the capillaries, the capillary attraction is sufficient to produce the mo- 

 tionless layer, called the " still layer " by many physiologists, and the liquid moves only 

 in the central portion. The plasma occupies the position next the walls of the vessels, 

 for it is this portion of the blood which is capable of " wetting " the tubes. The trans- 

 does not interfere with the circulation, and to attach the animal by pins to a thin piece of cork, stretching the web over 

 an orifice in the cork, to allow the passage of light, and securing it with pins through the toes. The membrane is 

 then moistened with water and covered with thin glass, and, if the general surface be kept moist, the circulation may 

 be studied for hours. By gently inflating the lungs with a small blow-pipe, securing the air by a ligature passed 

 around the larynx beneath the mucous membrane, and opening the chest, the pulmonary circulation may be studied. 

 The circulation may be examined in the tongue (which presents a magnificent view of the circulation as well as of the 

 nerves and muscular fibres) by drawing it out of the mouth and spreading it into a thin sheet, securing it with pins. 

 The circulation may also be observed in the mesentery of a small, warm-blooded animal, like the mouse, by fixing it 

 upon the frog- plate, opening the abdomen, and drawing out the membrane ; but it is not seen so well or so convenient- 

 ly as in the tongue or web of the frog. 



