THE CIRCULATION. 301 



other, or even in opposite directions ; penetrating everywhere the sub- 

 stance of the organ with a kind of vascular irrigation. 



The motion of the red and white globules is also peculiar, and shows 

 distinctly the difference in their physical properties. In the larger ves- 

 sels the red globules are carried along in close column, in the central 

 part of the stream ; while near the edges there is a transparent space 

 occupied by clear plasma, in which no red globules are visible. In 

 the smaller vessels the globules pass two by two, or follow each other 

 in single file. Their flexibility and semi-fluidity are very apparent as 

 they are folded, bent or twisted, or made to glide through branches of 

 communication, smaller than themselves. The white globules, on the 

 other hand, move more slowly. They drag along the external portions 

 of the current, and are sometimes arrested for a few seconds, adhering 

 to the inner surface of the vessel. Wherever the current is obstructed 

 or retarded, the white globules accumulate and become for the time 

 more numerous in proportion to the red. 



It is during its passage through the capillaries that the blood serves 

 for the nourishment of the tissues, and for absorption, secretion, or 

 elimination. The tenuity of the vascular walls, their extent of surface 

 in proportion to the blood which they contain, and the multiplication 

 of the currents due to their division and inosculation, all contribute to 

 this result, and make these vessels the most important physiological 

 division of the circulatory system. The nutritious ingredients of the 

 blood transude through their walls, and are appropriated by the tissues 

 beyond. In the glandular organs they supply the substances requisite 

 for secretion ; in the villi of the intestine they take up the elements of 

 the digested food ; in the lungs they absorb oxygen and exhale carbonic 

 acid ; and in the kidneys they discharge the products of destructive 

 assimilation, collected from other parts. The capillary circulation thus 

 furnishes, directly or indirectly, the materials for the growth and 

 renovation of the entire body. 



Physical Cause of the Capillary Circulation. The conditions 

 which influence the movement of the blood in the capillaries are some- 

 what different from those of the arterial and venous circulations. By 

 the successive division of the arteries from the heart outward, the 

 movement of pulsation is to a great extent equalized in their smaller 

 branches. But in the neighborhood of the capillary system, they sud- 

 denly break up into a terminal ramification of still smaller vessels, and 

 so lose themselves at last in the capillary network. By this final in- 

 crease of the vascular surface, the equalization of the heart's action is 

 completed. There is no longer any pulsating character in the force 

 which acts on the circulating fluid ; and the blood moves through the 

 capillary vessels under a continuous and uniform pressure. 



This pressure is sufficient to propel the blood through the capillary 

 plexus into the veins. The fact was first demonstrated by Sharpey, 

 who employed an injecting syringe with a double nozzle, one of its 

 extremities being connected with a mercurial gauge, while the other 



