THE CAPILLARIES. 93 



disappearance of the arterial muscle-cells marks the beginning of the true 

 capillaries. The passage of the latter into the veins is less definite, since 

 muscular tissue is wanting in both the capillaries and the smaller veins. 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. Although preformed openings (stomata) in the walls of the 

 capillaries do not exist, the escape of leucocytes (diapedesis) and, perhaps, 

 of minute particles of foreign substances, takes place between the endothelial 

 plates, under conditions favorable to congestion and increased blood-pressure. 

 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 syncy- 



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FIG. 130. Capillaries arising from arteriole and ending in small vein ; from the omentum. X 150. 



tium. When capillaries course in dense fibrous tissues, not uncommonly 

 the vessel is accompanied by ensheathing delicate strands of connective 

 tissue, the adventitia capillaris. 



In certain organs, conspicuously in the liver, the ultimate blood-vessels 

 arise by the invasion and subdivision of the larger primary blood-channel by 

 the developing tissue-cords. Such blood-vessels are known as sinusoids 

 (Minot) and differ from ordinary capillaries in connecting entering (afferent) 

 and emerging (efferent) vessels of the same nature, both being always venous 

 in character. Capillaries, on the contrary, establish communication between 

 arteries and veins. In consequence of the invagination and intergrowth 

 which takes place between the original blood-channel and the tissue of the 

 developing organ, the endothelium of the sinusoids has an unusually intimate 

 relation to the cords of tissue-cells, little or no connective tissue intervening. 



The capillaries are arranged usually as networks, the component chan- 

 nels of which are of fairly constant diameter within a particular tissue. 

 During life, it is probable 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 8 and 20 //. The capillary 

 networks in various parts of the body differ in the form and closeness of their 

 meshes, since these details are influenced by the arrangement of the elements 

 and by the function of the structures supplied. Thus, in muscles, tendons 

 <ir.d nerves the meshes are elongated and narrow; in glands, the lungs and 



