CIRCULATION OF THE BLOOD. 539 



in this peripheral layer is due to two causes. The white blood-cells are 

 much more adhesive than the red, and therefore tend to cling to the 

 sides of the capillaries. In addition to this, the white blood-cells are 

 lighter in specific gravity than the red, and it has been noticed that when 

 a fluid holding particles in suspension of two different densities is forced 

 through a capillary tube, the heavier particles will always pass through 

 the rapidly moving axial current, while the lighter particles will be in a 

 current by the sides of the tube, where the friction is greatest and where 

 motion is therefore slowest. 



When the circulation is studied in the vessels of the mesentery of 

 a warm-blooded animal, or where inflammation is produced in the tissues 

 of the web of a frog's foot by mechanical irritation, the corpuscles may 

 frequently be observed to pass through the walls of the vessel in great 

 numbers (diapedesis). At first the colorless cells are found to move 

 more and more slowly ; several accumulate and adhere to the wall and 

 ultimately pass out through it, during the act of passing being finely 

 drawn out into slender, protoplasmic threads. It is doubtful whether 

 actual stomata or openings exist between the cells which compose the 

 vascular walls, or whether they simply pass through the cement substance 

 between the endothelial cells. 



6. The Circulation in the Veins. — The veins are much less elastic 

 than the arteries, and so do not remain open, even in a dead body, after 

 the blood has been withdrawn ; otherwise they resemble the arteries in 

 structure, although the muscular element in them is unequally distributed. 

 The veins are, nevertheless, contractile, although unequally so, and may 

 often be noticed to reduce in diameter when the part is exposed to the 

 cold or to Various other irritations. The veins are very dilatable, and 

 are in capacity much greater than that of the arterial system : the veins 

 are, in fact, capable of containing the entire blood of the body. 



When a vein is cut the flow from the divided extremity occurs 

 usually from the distal end; that is, the one nearest the capillaries, alone. 

 It is continuous and of comparatively slight velocity. The pressure of 

 the blood within the veins, as determined by connecting a manometer 

 with them, is always much lower than in the arteries, and decreases as 

 the heart is approached, where during inspiration even a negative 

 pressure may be noticed. This is proved by the constant entrance of 

 the lymph, which is itself moved under an extremely low pressure, into 

 the large, venous trunks at the root of the neck. In the sheep the mean 

 pressure in the brachial vein has been found to be four millimeters of 

 mercury ; in the crural, eleven and four-tenths millimeters ; in the axillary 

 the pressure is usually negative, becoming one millimeter negative 

 during inspiration, and three to five millimeters during strong inspira- 

 tion, and becoming positive only during forced expiration. No pulse is 



