GIF. XVIII. j 



THE CAPILLARIES 



219 



in the coats of both arteries and veins. In the external coat of large 

 vessels they form a plexus of more or less tubular vessels. In smaller 

 vessels they appear as spaces lined by endothelium. Sometimes, as 

 in the arteries of the omentum, mesentery, and membranes of the 

 brain, in the pulmonary, hepatic, and 

 splenic arteries, the spaces are con- 

 tinuous with vessels which distinctly 

 ensheath them perivascular lym- 

 phatics (fig. 231). 



The Capillaries. 



In all vascular textures except 

 some parts of the corpora cavernosa 

 of the penis, of the uterine placenta, 

 and of the spleen, the transmission 

 of the blood from the minute branches 

 of the arteries to the minute veins is 

 affected through a network of capil- 

 laries. 



Their walls are composed of endo- 

 thelium a single layer of elongated 

 flattened and nucleated cells, so joined 

 and dovetailed together as to form a 

 continuous transparent membrane 

 (fig. 232). Here and there the endo- 

 thelial cells do not fit quite accu- 

 rately; the space is filled up with 

 cement material; these spots are 

 called pseudo-stomata. 



The diameter of the capillary 

 vessels varies somewhat in the 

 different tissues of the body, the 

 most common size being about 

 innnjth of an inch (12 /m). Among 

 the smallest may be mentioned 

 those of the brain, and of the fol- 

 licles of the mucous membrane of 

 the intestines; among the largest, 

 those of the skin, lungs, and especially those of the medulla of 

 bones. 



The size of capillaries varies necessarily in different animals in 

 relation to the size of their blood corpuscles : thus, in the Proteus, 

 the capillary circulation can just be discerned with the naked eye. 



The form of the capillary network presents considerable variety 



Fio. 228. Transverse section through a 

 small artery and vein of the mucous 

 membrane of a child's epiglottis ; the 

 artery is thick- walled and the vein thin- 

 walled. A. Artery, the letter is placed 

 in the lumen of the vessel, e. Endo- 

 thelial cells with nuclei clearly visible ; 

 these cells appear very thick from the 

 contracted state of the vessel. Outside 

 it a double wavy line marks the elastic 

 layer of the tunica intima. m. Tunica 

 media, consisting of unstriped muscular 

 fibres circularly arranged ; their nuclei 

 are well seen. o. Part of the tunica 

 adventitia showing bundles of connec- 

 tive-tissue fibre in section, with the 

 circular nuclei of the connective-tissue 

 corpuscles. This coat gradually merges 

 into the surrounding connective tissue. 

 v. In the lumen of the vein. The other 

 letters indicate the same as in the 

 artery. The muscular coat of the vein 

 (m) is seen to be much thinner than 

 that of the artery. x 350. (Klein 

 and Noble Smith.) 



