SECT. 215.] LARGEST VEINS. 497 



these vessels is, as before stated, undeveloped or even absent. The 

 error, however, could be readily avoided by tracing the condition 

 of the parts from the smaller veins onwards. The contractile 

 elements of the tunica adventitia are o - 02'" to 0*04'" in length, 

 and present the usual characters; besides this muscular layer 

 and the numerous longitudinal elastic networks, the outer coat 

 of the largest veins invariably contains a certain tpiantity of 

 connective tissue, which appears always to run in a transverse 

 direction ; so that even in these large veins the transverse elements 

 are not unrepresented, although they no longer exhibit muscular 

 fibres as their chief components. All the large veins which open 

 iuto the heart possess, for a short distance, an outer circular layer 

 of the same kind of muscular fibres which occurs in the heart, 

 characterised here also by anastomoses of the primitive bundles. 

 According to Rauschel, they extend along the upper vena cava as 

 far as the subclavian vein, and are also to be found upon the main 

 branches of the pulmonary veins; in the former situation, ac- 

 cording to Schrant, they are longitudinal, and are found even 

 towards the interior of the coat of the vessel. 



We have still to make special mention of certain veins, in which 

 the muscular elements are excessively developed, and of others in 

 which they are entirely absent. To the former class belong the 

 veins of the pregnant uterus, in which, besides the tunica media, 

 the inner and the outer coats also present longitudinal muscular 

 layers, whose elements exhibit the same colossal development in 

 the fifth and sixth months of pregnancy as those of the uterus 

 itself. The veins which are wholly destitute of muscular fibres 

 are : 1. The veins of the maternal portion of the placenta, in whose 

 Avails large cells and fibres occur external to the epithelium, which 

 I regard as undeveloped connective tissue. 2. The majority of the 

 veins of the cerebral substance and pia mater. These vessels are 

 made up of a simple layer of roundish epithelium, a thin longi- 

 tudinal layer of connective tissue with separate longitudinal nuclei, 

 representing the tunica media, and lastly, a tunica adventitia, 

 more homogeneous in the smaller vessels, but fibrillated and 

 nucleated in the larger. It is only rarely that a slight indication 

 of muscular fibres is exhibited, and then it is in the middle coat 

 of the largest of these veins, as represented in fig. 197. 3. The 

 sinuses of the dura mater and the veins of Breschet in the bones. 

 These are lined by a pavement-epithelium, and possess, external to 

 it, a layer of connective tissue, sometimes with fine elastic fibres, 

 which passes continuously into the tissue of the dura mater and 



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