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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
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 subjected 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 trabeculæ pushing their way into a large bloodvessel or blood space and carrying its endothelium 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 composed 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 endothelial, the middle the muscular, and the external the connective tissue or areolar (Fig. 452). The main difference between the veins and the arteries is in the comparative weakness of the middle coat in the former.


FIG. 451– Capillaries from the mesentery of a guinea-pig, after treatment with solution of nitrate of silver. a. Cells. b. Their nuclei. (See enlarged image)

  In the smallest veins the three coats are hardly to be distinguished (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 membrane (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 Kölliker, 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 connective 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 connective 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-mentioned 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

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