STRUCTURE OF BLOOD-VESSELS. 



679 



Fig. 641. 



i 



i 





stream and the passage of the plasma and oxygen into the surrounding tissues. 

 The walls of the capillaries consist of only the lining plates, the entire vessel being in 

 fact a delicate endothelial tube. The cells composing the latter are 

 elongated lanceolate plates, possessing oval nuclei, united by nar- 

 row lines of cement substance. Although the transition from the 

 arterioles is gradual, the final disappearance of the muscle-cells marks 

 the beginning of the true capillaries ; the passage of the latter into 

 the veins is less certain, since muscular tissue is wanting in those 

 of small size. In the smallest capillaries two endothelial plates may 

 suffice to encircle the entire lumen ; in the larger three or four cells 

 may be required to complete the vessel. Preformed openings (sto- 

 mata) in the walls of the capillaries do not exist, the passage of the 

 leucocytes and, under certain conditions, also of the red blood-cells 

 (diapedesis) and of small particles of foreign substances, being effected 

 between the endothelial plates. In some capillaries, as in those of 

 the choroid, liver, or renal glomeruli, the usual demarcation of the 

 wall into distinct cells is wanting, the individual endothelial plates 

 being replaced by a continuous nucleated sheet or syncytial layer. 

 Where the capillaries course within fibrous tissue, not uncommonly the 

 vessel is accompanied by delicate strands of connective tissue i^adveiititia capillaris) 

 that suggest an external sheath. 



The capillaries are usually arranged as net-works, of which the channels are of 

 fairly constant size within the tissue to which they are distributed. During life it is prob- 

 able that none are too small to permit the passage of the red blood-cells, while many 

 admit two or even three such elements abreast. Their usual diameter varies between 

 .008 and .020 mm. The capillary net- works in various parts of the body differ in the 

 form and closeness of their meshes, since these details are influenced by the arrange- 

 ment of the component elements and by the function of the structures supplied. Thus, 

 in muscles, tendons, and nerves the meshes are elongated and narrow; in glands, the 

 lungs, and adipose tissue they are irregularly polygonal; in the liver-lobules converg- 

 ingly or radially disposed; while in the subepithelial papillae of the mucous membranes 

 and the skin the capillaries commonly form loops. In general, it may be assumed that 



Portion of fem- 

 oral vein, opened 

 to show bicuspid 

 valve. 





Fig. 642. 





.*«♦* 



Capillary 



Arteriole ^^ i? I ' e /, ^^ - 'I^^J^'^jb^^ 



i^'w^f^^^r^^^^^^.im .: m 





vr 







vein 



Capillaries arising from arteriole and ending in small vein in omentum. X 200. 



the greater the functional activity of an organ, the closer is its capillary net- work. 

 Organs actively engaged in excretion, as the kidneys, or the elimination of substances 



