THE CIRCULATION OF THE BLOOD. 



323 



Functions of Capillary Vessels. The structure of the capil- 

 lary vessel is such as to permit the blood to come into intimate re- 

 lation with the tissues ; consisting of only a single layer of endothelial 

 cells, a free exchange between blood "and tissues at once takes place. 

 The nutritive materials of the blood pass through the capillary wall 

 into the tissue spaces; the waste products, the result of tissue metabo- 

 lism, pass through the capillary wall and into the blood, and by it are 

 carried to the eliminating organs. As a result of these exchanges 

 the relative composition of the blood is changed. It now contains 

 a smaller quantity of nutritive material and a larger quantity of 

 waste materials; containing less oxygen, it changes in color from red 

 to bluish-red. 



Though the length of the capillary is not more than 0.5 mm. and 

 the time occupied by the blood in passing through it not more than 

 0.8 second, yet the conditions are such that an exchange of materials 

 sufficient for nutritive purposes takes place. It is for this reason that 

 the rest of the circulatory apparatus exists.- Collectively its different 

 parts cooperate to give to the blood in the capillaries the volume, 

 the^ velocity, and the pressure requisite for nutritive purposes. 



Migration of the White Corpuscles. A phenomenon fre- 

 quently observed in the 

 capillary vessels of the 

 mesentery or of the blad- 

 der of the frog is the pas- 

 sage of the white corpuscles 

 through the walls into the 

 surrounding lymph-spaces. 

 To this process the term 

 migration or diapedesis is 

 given. After the tissues 

 have been exposed to the 

 air for some time or sub- 

 jected to an irritant, the 

 vessels dilate and become 

 distended with blood. In 

 a short time the blood- 

 stream slows, and finally 

 comes to rest. The condi- 

 tion of stasis is then estab- 

 lished. During the development of this condition the white corpus- 

 cles accumulate in large numbers along the inner surface of the 

 vessels and soon begin to pass through the vessel- walls. This they 

 do by the protrusion of a portion of their substance which is inserted 

 into and through the vessel- wall. This once accomplished, the re- 

 mainder of the cell in due time follows until it has entirely passed 



FIG. 155. SMALL VESSEL OF A FROG'S MESEN- 

 TERY SHOWING DIAPEDESIS. TV, -w. Vas- 

 cular walls, a, a. Poiseuille's space, r, r. 

 Red corpuscles. /, /. Colorless corpuscles 

 adhering to the wall, and c, c, in various 

 stages of extrusion. /, /. Extruded cor- 

 puscles. (Landois and Stirling.) 



