CHAP. XXVIII.] CAPILLARY VESSELS. 327 



the vessel, and the internal layer or fenestrated membrane which, 

 in every respect, corresponds to the internal longitudinal fibrous 

 layer of arteries. Upon this is placed the epithelium, which is 

 precisely the same as that of arteries. 



The imperfect development in veins of a tunic possessing much 

 elastic power, like the circular fibrous coat of arteries, explains the 

 readiness with which these vessels collapse, and the general thin- 

 ness of the fibrous and areolar tunics accounts for the diaphanous 

 character of the venous wall. 



In a large proportion of the venous system peculiar processes, called 

 valves, are found projecting into the interior of the vessel at various 

 points. These processes are semilunar, attached by their convex 

 border to the wall of the vein, and free at their concave border, 

 which is a little thickened. They are disposed in pairs in imme- 

 diate juxtaposition sometimes there are three placed together. 

 They are most numerous in the superficial or subcutaneous veins : 

 and are more so in the veins of the lower half of the body than in 

 those of the upper. The smallest veins are destitute of valves ; as 

 also are the largest, as the cavse. The pulmonary veins, those of 

 the liver, and all the veins which contribute to the hepatic portal 

 system, the splenic and meseiiteric veins want valves. The renal 

 veins are also devoid of them. 



The tissue of which these valves are composed is the same as 

 the longitudinal fibrous coat of the vein, covered by a layer of epi- 

 thelium; the valves cannot be properly described as reduplications 

 of the inner membrane of the veins, they are processes of it.* 



Of the Capillaries. The system of vessels which is intermediate 

 to the veins and arteries, is called by the name capillary, from the 

 minuteness of their size. The finest arteries, and the finest veins 

 likewise, receive this appellation. But the true capillary system is 

 distinguished by a speciality of arrangement and an uniformity of 

 size, proper to each tissue or organ. 



The capillary vessels may be examined in injected specimens, or 

 in recent transparent tissues, as in the pia-mater, or in living trans - 

 parent tissues, as the web of the frog's foot, the mesentery or 

 distended urinary bladder of the frog, the tail of the newt, the 

 gill of the tadpole, the tail or fins of fishes. 



The diameter of the capillaries varies in different textures from 



* We have great pleasure in referring to an excellent article on the anatomy 

 of veins in the 42nd part of the Cyclopaedia of Anatomy and Physiology, by 

 Dr.S.J.Salter. 



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