156 THE ORGANS 



the living condition the lining of the capillary is probably nearly 

 smooth. According to some investigators a delicate cuticle limits 

 the cytoplasm on the side toward the lumen. The edges of the 

 cells are serrated and are united by a small amount of intercellular 

 substance (p. 8i), which can be demonstrated by the silver- nitrate 

 stain. ^ In certain capillaries — those of the early embryo, of the 

 kidney glomeruli, of the chorioid coat of the eye, of the liver — no cell 

 boundaries can be made out. In these capillaries the endothelium 

 appears to be of the nature of a syncytium (p. 70). Capillaries 

 branch without diminution in calibre, and these branches anasto- 



I 



Fig. 92. — ^Large and Small Capillaries. Silver-nitrate and hsematoxylin stain (technic 

 7, p. 79), to show outlines of endothelial cells and their nuclei. 



mose to form capillary networks, the meshes of which differ in size 

 and shape in different tissues and organs (Figs. 92, 93, 94). The 

 largest meshed capillary networks are found in the serous membranes 

 and in the muscles, while the smallest are found in the glands, as, e.g., 

 the liver. The largest capillaries are found in the liver, the smallest 

 in muscles. 



In such thin membranous parts as the web of the frog's foot, or the wall of 

 the frog's bladder, the blood may be observed as it flows through the arteries, 

 capillaries, and veins. The current is seen to be fastest in the arteries, and 

 faster in the centre of the vessel than at its periphery. It is slower in the veins 

 and slowest in the capillaries. In the case of the frog's bladder, the mere expo- 

 sure to the air acts as a sufficient irritant to cause slight inflammatory changes, 

 and the leucocytes may be seen adhering to the waUs of the capillaries and 

 passing through them into the tissues. The capillary, both from the thinness of 

 its wall and from the slowness with which the blood passes through it, is peculiarly 

 adapted for the interchange of material between the blood and the tissues, and 

 it is probable that it is in the capillary that all such interchange takes place. 



^ Some authors describe delicate protoplasmic anastomoses between the cells and 

 an irregular precipitation of the silver nitrate corresponding to the spaces between the 

 anastomosing threads- These spaces are interpreted as intercellular channels through 

 which the leucocytes and plasma pass. Intracellular spaces reacting to silver nitrate 

 and supposed to have a similar function have also been described. 



