CIRCULATION OF THE BLOOD IN THE VESSELS. 



muscular coat, and, indeed, the action of vessels of considerable size. This 

 has been a valuable means of studying the circulation in the capillaries as 

 contrasted with the flow in the small arteries and veins, and the only one, 

 indeed, which could give any definite idea of the action of these vessels. 



In studying the circulation under the microscope, the anatomical division 

 of the blood into corpuscles and a clear plasma is observed. This is peculiarly 

 evident in cold-blooded animals, the corpuscles being comparatively large and 

 floating in a plasma which forms a distinct layer next the walls of the vessel. 



The leucocytes, which 



, are much fewer than 



the red corpuscles, are 

 generally found in the 

 layer of plasma. 



In vessels of consid- 

 erable size as well as in 

 some capillaries, the cor- 

 puscles, occupying the 

 central portion, move 

 with much greater ra- 

 pidity than the rest of 

 the blood, leaving a lay- 

 er of clear plasma at 

 the sides, which is near- 

 ly motionless. This 

 phenomenon is in obe- 

 dience to a physical 

 law regulating the pas- 

 sage of liquids through 

 capillary tubes for which they have an attraction, such as exists, for exam- 

 ple, between the blood and the vessels. In tubes reduced to a diameter ap- 

 proximating that of the capillaries, the attractive force exerted by their walls 

 upon a liquid, causing it to enter the tube to a certain distance, 'becomes an 

 obstacle to the passage of fluid in obedience to pressure. Of course, as the 

 diameter of the tube is reduced, this force becomes relatively increased, for 

 a larger proportion of the liquid contents is brought in contact with it. In 

 the smallest arteries and veins, and still more in the capillaries, the capillary 

 attraction is sufficient to produce the motionless layer, sometimes called the 

 " still layer," and the liquid moves only in the central portion. The plasma 

 occupies the position next the walls of the vessels, for it is this portiSn of the 

 blood which is capable of " wetting " the tubes. The transparent layer was 

 observed by Malpighi, Haller and all who have described the capillary circu- 

 lation. Poiseuille recognized its true relation to the blood-current and ex- 

 plained the phenomenon of the still layer by physical laws, which had been 

 previously established with regard to the flow of liquids in tubes of the di- 

 ameter of one twenty-fifth to one one-eighth of an inch (1 to 3'2 mm.), but 

 which he had succeeded in applying to tubes of the size of the capillaries. 



FIG. 34. Circulation in the web of the frog's foot (Wagner). 

 The black spots, some of them star-shaped, are collections of pigme 

 a, a venous trunk, composed of three principal branches (&, 6, 

 and covered with a plexus of smaller vessels (c, c). 



ent. 



