THE CIRCULATION. 



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 of the vessel there is a trans- 

 parent space occupied only by clear plasma, in which no red globules 

 are to be seen. In the smaller vessels the globules pass two by two, or 

 follow each other in single file. The flexibility and semi-fluid consist- 

 ency of the red globules are very apparent from the readiness with 

 which they become folded up, bent or twisted, and with which they 

 glide through minute branches of communication, smaller in diameter 

 than themselves. The white globules, on the other hand, move more 

 slowly through the vessels. They drag along the external portions of 

 the current, and are sometimes temporarily arrested, adhering for a few 

 seconds to the internal surface of the vessel. Whenever the current is 

 obstructed or retarded, the white globules accumulate in the affected 

 portion, and become more numerous there in proportion to the red. 



It is during the capillary circulation that the blood serves for the 

 nutrition of the vascular organs. Its fluid portions transude through 

 the walls of the vessels, and are absorbed by the tissues in the propor- 

 tions requisite for their nourishment, or for the products of secretion ; 

 while its albuminous ingredients are also transformed into new materials, 

 characteristic of the different tissues and fluids. In this way are pro- 

 duced the myosine of the muscles, the collagen of the bones, tendons, 

 and ligaments, the ptyaline of the saliva, and the pepsine of the gastric 

 juice ; and in the lungs, the exchange of oxygen and carbonic acid takes 

 place in the capillary vessels. The blood in the capillary circulation 

 thus furnishes, directly or indirectly, the materials of nutrition for the 

 entire body. 



Physical Cause of the Capillary Circulation. The physical condi- 

 tions which influence the movement of the blood in the capillaries are 

 somewhat 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 the smaller 

 arterial branches. But as these vessels reach the confines of the capillary 

 system, they suddenly break up into a terminal ramification of still 

 smaller and more numerous vessels, and so lose themselves at last in 

 the capillary network. By this final increase 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 upon the circulating 

 fluid ; and the blood moves through the capillary vessels under a con- 

 tinuous and uniform pressure. 



This pressure is sufficient to cause the blood to pass with considerable 

 rapidity through the capillary plexus, into the commencement of the 

 veins. This fact was first demonstrated by Sharpey, 1 who employed an 

 injecting syringe with a double nozzle, one extremity of which was con- 



1 Todd and Bowman, Physiological Anatomy and Physiology of Man, vol. ii. 

 p. 350. 



