503 THE TISSUES. 



the inner half, or two-thirds of the external coat, which rests upon the 

 tunica intima directly, and may become ^ of an inch thick. Kol- 

 liker found these longitudinal bundles extending through the entire 

 thickness of the external coat in the renal veins and the vena portse ; 

 and very well developed also in the trunks of the hepatic veins, 

 the remaining portions of the vena cavas, and to the splenic, supe- 

 rior mesenteric, and external iliac veins. All the large veins open- 

 ing into the heart, are furnished for a short distance also with an 

 external annular layer of striated muscular fibres, like those of the 

 heart itself, and some of them also presenting anastomoses. They 

 extend in the superior vena cavas to the subclavian vein ; and may 

 be found in the main branches of the pulmonary veins. (Rauschel?) 



The tunica media of the large veins is sparingly developed, and 

 especially its muscular elements, which are, however, often abu,nd- 

 ant in the external coat, as has just been stated. It is usually g j^ 

 to 3^ of an inch thick; but may be 3 J to T ^ of an inch (at the 

 orifices of the hepatic veins), or may be wholly wanting; as in the 

 greater part of the vena cava in the liver, and the trunks of the 

 hepatic veins. Its longitudinal elastic networks are intimately con- 

 nected together, and less distinctly laminated than in the medium- 

 sized veins, or not at all so. The transverse muscular fibres are 

 also scanty and less distinct; being most developed in the splenic 

 vein arid vena portaa. The tunica intima is usually T5 Vu f an inch, 

 when it presents the same conditions as in the medium-sized veins. 

 . Karely, as in the vena cava inferior, the trunk of the hepatic, and 

 in the vena innominata, it is g ^ to ^<y of an inch thick; this in- 

 crease of thickness being due to striped lamellm with nuclei, and 

 fine elastic longitudinal networks, and never to lamella composed 

 of muscular fibres. 



In the veins of the gravid uterus the muscular element is exclu- 

 sively developed; it existing in all three of their tunics. It is en- 

 tirely wanting, 1, in the veins of the maternal portion of the pla- 

 centa; 2, in most of the veins of the cerebral substance and of the 

 pia mater; 3, in the sinuses -of the dura mater, and the veins of 

 Breschet in the bones (which have merely a layer of collagenous 

 tissue external to a scaly epithelium); 4, in the sinuses of the cor- 

 pora cavernosa; and 5, in the veins of the retina. 



The valves of the veins are projections of the middle and internal 

 tunics. Muscular fibres have been found by Wahlgren in the 

 larger valves. 



