

STRUCTURE OF VEINS 



501 



each consists of two parts: of hyaline ground plates, and of a protoplasmic granular part, in 

 which is imbedded the nucleus, on the outside of the ground plates. The hyaline internal coat 

 of the capillaries does not form a complete membrane, but consists of "plates" which are inelastic, 

 and though in contact with each other are not continuous; when therefore the capillaries are sub- 

 jected to intravascular pressure, the plates become separated from each other; the protoplasmic 

 portions of the cells, on the other hand, are united together. In some organs, e. g., the glomeruli 

 of the kidneys, intercellular cement cannot be demonstrated in the capillary wall and the cells 

 are believed to form a syncytium. 



In many situations a delicate sheath or envelope of branched nucleated connective tissue cells 

 is found around the simple capillary tube, particularly in the larger ones; and in other places, 

 especially in the glands, the capillaries are invested with retiform connective tissue. 



Sinusoids. In certain organs, viz., the heart, the liver, the suprarenal and parathyroid 

 glands, the glomus caroticum and glomus coccygeum, the smallest bloodvessels present various 

 differences from true capillaries. They are wider, with an 

 irregular lumen, and have no connective tissue covering, 

 their endothelial cells being in direct contact with the cells of 

 the organ. Moreover, they are either arterial or venous and 

 not intermediate as are the true capillaries. These vessels 

 have been called sinusoids by Minot. They are formed 

 by columns of cells or trabeculse pushing their way into a 

 large bloodvessel or blood space and carrying its endothe- 

 lium before them ; at the same time the wall of the vessel 

 or space grows out between the cell columns. 



Structure of Veins. The veins, like the arteries, are com- 

 posed of three coats: internal, middle, and external; and 

 these coats are, with the necessary modifications, analogous 

 to the coats of the arteries; the internal being the endo- 

 thelial, the middle the muscular, and the external the 

 connective tissue or areolar (Fig. 452). The main differ- 

 ence between the veins and the arteries is in the compara- 

 tive weakness of the middle coat in the former. 



In the smallest veins the three coats are hardly to be dis- 

 tinguished (Fig. 449). The endothelium is supported on a 

 membrane separable into two layers, the outer of which 

 is the thicker, and consists of a delicate, nucleated mem- 

 brane (adventitia) , while the inner is composed of a network 

 of longitudinal elastic fibers (media). In the veins next 

 above these in size (0.4 mm. in diameter), according to 

 Kolliker, a connective tissue layer containing numerous 

 muscle fibers circularly disposed can be traced, forming the 

 middle coat, while the elastic and connective tissue elements of the outer coat become 

 more distinctly perceptible. In the middle-sized veins the typical structure of these vessels 

 becomes clear. The endothelium is of the same character as in the arteries, but its cells 

 are more oval and less fusiform. It is supported by a connective tissue layer, consisting of 

 a delicate net-work of branched cells, and external to this is a layer of elastic fibers disposed 

 in the form of a net-work in place of the definite fenestrated membrane seen in the arteries. 

 This constitutes the internal coat. The middle coat is composed of a thick layer of con- 

 nective tissue with elastic fibers, intermixed, in some veins, with a transverse layer of muscular 

 tissue. The white fibrous element is in considerable excess, and the elastic fibers are in much 

 smaller proportion in the veins than in the arteries. The outer coat consists, as in the arteries, 

 of areolar tissue, with longitudinal elastic fibers. In the largest veins the outer coat is from 

 two to five times thicker than the middle coat, and contains a large number of longitudinal 

 muscular fibers. These are most distinct in the inferior vena cava, especially at the termination 

 of this vein in the heart, in the trunks of the hepatic veins, in all the large trunks of the portal 

 vein, and in the external iliac, renal, and azygos veins. In the renal and portal veins they extend 

 through the whole thickness of the outer coat, but in the other veins mentioned a layer of con- 

 nective and elastic tissue is found external to the muscular fibers. All the large veins which open 

 into the heart are covered for a short distance with a layer of striped muscular tissue continued 

 on to them from the heart. Muscular tissue is wanting: (1) in the veins of the maternal part 

 of the placenta; (2) in the venous sinuses of the dura mater and the veins of the pia mater of 

 the brain and medulla spinalis; (3) in the veins of the retina; (4) in the veins of the cancellous 

 tissue of bones; (5) in the venous spaces of the corpora cavernosa. The veins of the above-men- 

 tioned parts consist of an internal endothelial lining supported on one or more layers of areolar 

 tissue. 



Most veins are provided with valves which serve to prevent the reflux of the blood. Each 

 valve is formed by a reduplication of the inner coat, strengthened by connective tissue and elastic 



FIG. 451. -^-Capillaries from the mesen- 

 tery of a guinea-pig, after treatment with 

 solution of nitrate of silver, a. Cells. 

 6. Their nuclei. 





