66 CIRCULATION OF THE BLOOD 



The most convenient part for direct observation of the capillary 

 circulation is the tongue or the web of the frog. Here may be studied, 

 not only the movement of the blood in the capillaries, but the circula- 

 tion in the smallest arteries and veins, the variations in calibre of these 

 vessels, especially the arterioles, by the action of their 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 con- 

 trasted with the flow in the small arteries and veins. 



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 that forms a distinct 

 layer next the walls of the vessel. The leucocytes, which are much 

 fewer than the red corpuscles, are found usually in the layer of 

 plasma. 



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

 corpuscles, occupying the central portion, move with greater rapidity 

 than the rest of the blood, leaving a comparatively still layer of plasma 

 at the sides. A red corpuscle occasionally becomes involved in the 

 "still layer," when it moves slowly, turning over and over, or even may 

 remain stationary for a time, until it is taken up again and carried 

 along with the central current. A few leucocytes are constantly seen 

 in this layer. They move along slowly and apparently have a ten- 

 dency to adhere to the walls of the vessel. This is due to the adhesive 

 character of the surface of the white corpuscles as compared with the 

 red, which can easily be observed in examining a drop of blood be- 

 tween glass surfaces, the red corpuscles moving about freely, while the 

 white corpuscles have a tendency to adhere to the glass. 



Great differences exist in the character of the flow of blood in the 

 three varieties of vessels under observation. In the arterioles, which 

 may be distinguished from the capillaries by their size and the 

 presence of the muscular and fibrous coats, the movement is distinctly 

 pulsatile, even in their most minute ramifications. The blood moves 

 in them with greater rapidity than in either the capillaries or veins. 

 They become smaller as they branch, and carry the blood always in 

 the direction of the capillaries. The veins, which are relatively 

 larger than the arteries, carry the blood more slowly and in a 

 continuous stream from the capillaries toward the heart. In both 

 the arteries and veins, the current is frequently so rapid that the 

 form of the corpuscles can not be distinguished. Only a few of the 

 leucocytes occupy the still layer, the others being carried on in the 

 central current. 



