CAPILLARIES. 



127 



FIG. 151. CAPILLARY PROM THE TAIL 

 OP A TADPOLE. SILVER NITRATE 

 PREPARATION. (After Koelliker.) 



CAPILLARIES are endothelial tubes of varying diameter, the smallest 

 being so narrow that the blood corpuscles are distorted in passing through 

 them in single file. Their walls are composed of elongated, very flat cells 

 with irregularly wavy margins as shown in Fig. 151, from a silver nitrate 

 preparation. Between the cells the corpuscles, both red and white, may 

 make their way out of the vessel. There 

 are no preformed openings for this purpose, 

 and the endothelial cells come together after 

 the corpuscles have passed out. Two cells 

 form the circumference of small capillaries, 

 4.5 to 7 u in diameter, and three or four 

 cells bound the larger ones of 8 to 13 //. 

 Nerves end in contact with them and it is 

 possible for the endothelial cells to contract. 

 The bulging of their nuclei into the lumen 



of the vessel, often seen in specimens of capillaries and of larger vessels, 

 is probably an artificial appearance. The lining in life is thought to be 

 smooth. Certain endothelial cells are said to be phagocytic, devouring 

 objects which float in the blood, and some endothelial cells have been 

 described as becoming detached and entering into the circulation. Small 

 capillaries divide without decrease in caliber, and by anastomosis with 



neighboring capillaries 

 they form networks differ- 

 ing widely in the size of 

 the meshes. The closest 

 meshes occur in the secre- 



K * ' 



~ '* ' -*" tory organs and in the 



lungs and mucous mem- 



&"! \ \ff~ 



ip \f i branes; the widest are 



fh r it / '' fir I * n musc ^ es ' tne serous 



membranes and the sense 

 organs. The close net- 

 works consist of capil- 

 laries of large caliber; and 

 those with wide meshes 

 are formed of more slen- 

 der vessels. Thus the blood supply of glandular organs is particularly 

 abundant. The sinusoids of the liver are close meshed and large. 



ARTERIES, in approaching their terminal branches, become small 

 (arterioles) and as ' precapillary vessels' pass without line of demarcation 

 into capillaries. The smallest arteries are endothelial tubes encircled 





. 



FIG. 152. SMALL ARTERIES OP MAN'. 



Nuclei of endothelial cells; m, nuclei of circular muscle fibers, 

 at m' seen in optical cross section; a, nuclei of connective 

 tissue. In A, since the endothelium is out of focus, its 

 nuclei are not seen. X 240. 



