THE BLOODVESSELS. 689 



fasciculi. According to Riiuschel, these muscles, 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 ; and even, ac- 

 cording to Schrant, in the former case, more in the interior of the wall 

 of the vessels, and disposed longitudinally. 



The veins in which the muscular clement is excessively developed de- 

 mand 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 adventitia also present 

 muscular layers, longitudinal 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 sub- 

 stance, and of the pia mater. These veins consist of a roundish epithe- 

 lium in a single layer, a thin longitudinal connective 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, occa- 

 sionally containing fine elastic fibres, external to a tessellated epithe- 

 lium, and which layer is continuous with that of the dura mater and of 

 the internal periosteum. 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, containing 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 may, therefore, be regarded as continuations of the middle 

 and internal tunics, although muscular fibres, so far as I have seen, are 

 wanting in them (Wahlgren states that he has found such fibres in the 

 larger valves). 



217. Capillaries (vasa capillaria). With the solitary exception of 

 the corpora cavernosa of the sexual organs, and of the uterine placenta, 

 the arteries and veins, in Man, are universally connected by the inter- 

 vention of a rich plexus of microscopically fine vessels, which, on account 

 of their slender dimensions, have been designated under the above name, 



44 



