THE BLOOD-VESSELS. 307 



and which also present anastomoses of the primitive fasciculi. 

 According to Rauschel, these nluscles, in the region of the 

 superior vena cava, extend as far as to the subclavian vein, and 

 may be found also in the main branches of the pulmonary 

 veins j and even, according to Schrant, in the former case, more 

 in the interior of the wall of the vessels, and disposed longi- 

 tudinally. 



The veins in which the muscular element is excessively de- 

 veloped demand special notice, and also those in which that 

 element is wholly wanting. To the former class belong the 

 veins of the gravid uterus, in which besides the t. media, the 

 tt. intima and advenlitia also present muscular layers, longitu- 

 dinal in the two latter, the elements of which in the 5th and 

 6th month exhibit the same colossal development as those of the 

 uterus itself. The muscular element is wanting : 1. in the veins 

 of the maternal portion of the placenta, in whose walls, externally 

 to the epithelium, large cells and fibres, which I regard as 

 undeveloped connective-tissue, occur. 2. in most of the veins of 

 the cerebral substance, and of the pia mater. These veins con- 

 sist of a roundish epithelium in a single layer, a thin longitudinal 

 connective-tissue layer, with solitary elongated nuclei, which 

 supplies the place of the t. media, and in the smaller vessels of 

 a more homogeneous, and in the larger, of a fibrillated and 

 nucleated t. adventitia. It is but rarely that a faint indication 

 of muscles in the t. media is seen in the largest of these veins, 

 as shown in fig. 279. 3. in the sinuses of the dura mater and the 

 veins of Breschet in the bones, which are furnished with a layer 

 of connective tissue, occasionally containing fine elastic fibres, 

 external to a tesselated epithelium, and which layer is con- 

 tinuous with that of the dura mater and of the internal peri- 

 osteum. 4. in the venous sinuses of the corpora cavernosa (vid. 

 sup.), and of the spleen of certain Mammalia, (vid. § 169). 

 5. in the veins of the retina. 



The valves of the veins consist chiefly of distinct connective 

 tissue, which, at their free border, runs transversely, con- 

 taining numerous elongated nuclei, and also isolated, undulating, 

 usually fine, in part strong, elastic fibres. Their surface is 

 covered either with nothing but an epithelium, with short cells, 

 or in addition, there is beneath it, a very fine elastic network, 

 the prevailing direction of which is longitudinal. The valves 



