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The valves of the veins are reduplications of the intima and vary slightly in structure at their two surfaces. The inner surface next to the blood current is covered by elongated endothelial cells, while the outer surface possesses an endothelial lining composed of much shorter cellular elements. The greater part of the valvular structure consists of white fibrous connective-tissue and elastic fibers. Flattened and circularly arranged muscle-cells are met with at the inner surface of many of the larger valves. The elastic fibers are more numerous beneath the endothelium on the inner surface of the valves (Ranvier, 89).

(c) The Capillaries.-The capillaries consist solely of a layer of endothelial cells, accompanied here and there by a very delicate structureless membrane, and rarely by stellate connective-tissue cells. The connective tissue in the immediate neighborhood of the capillaries is modified to such an extent that its elements, especially those of a cellular nature, seem to be arranged in a direction parallel with the

Fig. 169. Endothelial cells of capillary (a) and precapillary (b) from the mesentery of rabbit; stained in silver nitrate.

long axis of the capillaries. When examined in suitable preparations, the endothelium of the capillaries is seen to form a continuous layer, the cells of which are, as a rule, greatly flattened and present very irregular outlines.

It is a well-known fact that a migration of the leucocytes occurs from the capillaries and smaller vessels (compare p. 175). In this connection arises the question as to whether or not the cells pass through certain preformed openings in the endothelium of these vessels, the so-called stomata, or through the stigmata and intercellular cement uniting the endothelial cells. The latter seems more probable, as stomata do not occur normally in the capillary wall. This subject will be further touched upon in the description of the lymphatic system.

The capillaries connect the arterial and venous precapillary vessels, and in general accommodate themselves to the shape of the elements of tissues or organs in which they are situated. In the

muscles and nerves, etc., they form a network with oblong meshes, while in structures having a considerable surface, such as the pulmonary alveoli, the meshes are more inclined to be round or oval; such small evaginations of tissue as the papillæ of the skin contain capillaries arranged in the shape of loops. In certain organs-as, for instance, in the lobules of the liver-the capillaries form a distinct network with small meshes.

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(d) Anastomoses, Retia mirabilia, and Sinuses. In the course of certain vessels, abrupt changes are seen to occur-as, for instance, when a small vessel suddenly breaks up into a network of capillary or precapillary vessels, which, after continuing as such for a short distance, again unite to form a larger blood-channel, the latter then dividing as usual into true capillaries. Such structures are known as retia mirabilia, and occur in man in the kid

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Fig. 170.-Small artery from the oral submucosa of cat, stained in methyleneblue, and showing a small portion of a sensory nerve-ending and the plexus of vasomotor

nerves.

ney, intestine, etc. Again, instead of breaking up into capillaries, a vessel may empty into a large cavity lined by endothelial cells (blood sinus). The latter is usually surrounded by loose connective tissue and is capable of great distention when filled with blood from an afferent vessel, or when the lumen of the efferent vessel is contracted by pressure or otherwise. The cavernous or erectile tissue of certain organs is due to the presence of such sinuses (penis, nasal mucous membrane, etc.). If vessels of larger caliber possess numerous direct communications, a vascular plexus is the result; but if such communications occur at only a few points, we speak of anastomoses. Especially important are the direct communications between arteries and veins without the mediation of capillaries. Certain structural conditions of the tissue appear to favor such anomalies, which occur in certain exposed

areas of the skin (ear, tip of nose, toes) and in the meninges, kidney, etc.

The blood-vessels, and more particularly the arteries, possess a rich nerve supply, comprising both nonmedullated and medullated nerves. The nonmedullated nerves, the neuraxes of sympathetic neurones, the cell-bodies of which are situated as a very general rule in some distant ganglion, form plexuses in the adventitia of the vessel-walls; from this, single nerve-fibers, or small bundles of such, are given off, which enter the media and, after repeated division, end on the involuntary muscle-cells in a manner previously described. (See p. 149 and Fig. 128.) Through the agency of these nerves, the caliber of the vessel is controlled. They are known as vasomotor nerves. Quite recently Dogiel, Schemetkin, and Huber have shown that many vessels possess also sensory nerve-endings. The medullated nerve-fibers terminating in such endings, branch repeatedly before losing their medullary sheaths. These nerve-fibers with their branches accompany the vessels in the fibrous tissue immediately surrounding the adventitia. The nonmedullated terminal branches end in telodendria, consisting of small fibrils, beset with large varicosities and usually terminating in relatively large nodules.

The branches and telodendria of a single medullated nerve-fiber (sensory nerve) terminating in a vessel are often spread over a relatively large area, some of the branches of such a nerve often accompanying an arterial branch, to terminate thereon. In the large vessels, the telodendria of the sensory nerves are found not only in the adventitia, but also in the intima, as has been shown by Schemetkin. (See p. 193.)

COOPER ME

COLLE E

B. THE LYMPHATIC SYSTEM.

1. LYMPH-VESSELS.

The larger lymph-vessels-the thoracic duct, the lymphatic trunks, and the lymph-vessels-have relatively thin walls, and their structure corresponds in general to that of the veins. They possess numerous valves, and are subject to great variation in caliber according to the amount of their contents. When empty, they collapse and the smaller ones are not easily distinguished from the surrounding connective tissue. Timofeew and Dogiel (97) have shown that the lymph-vessels are supplied with nerves, which in their arrangement are similar to those found in the arteries and veins, though not so numerous. The latter, who has given the fuller description, states that the nerves supplying the lymphvessels are varicose, nonmedullated fibers which form plexuses surrounding these structures. The terminal branches would appear to end on the nonstriated muscle cells found in the wall of the lymph-vessel.

2. LYMPH CAPILLARIES, LYMPH-SPACES, AND SEROUS CAVITIES. The walls of the lymph capillaries consist of very delicate, flattened endothelial cells, which are, however, somewhat larger and more irregular in outline than those of the vascular capillaries. The two may also be further differentiated by the fact that the diameter of the lymph capillaries varies greatly within very short distances. From a morphologic standpoint, the relations of the lymph capillaries to the vascular capillaries and adjacent tissues are among the most difficult to solve. The distribution of the lymph-vessels and capillaries can be studied only in injected preparations, and it is easily seen that structures of such elasticity and delicacy are peculiarly liable to injury by bursting under this method of treatment. The resulting extravasations of the injection-mass then spread out in the direction of least resistance and still further obscure the picture, rendering it difficult to determine what spaces are preformed and what are the result of the injection. So much is, however, certain that the more carefully and skilfully the injection is made, the greater are the areas obtained, showing the injection of true lymph capillaries.

In some regions very dense networks of lymph capillaries surrounding the smaller blood-vessels have been demonstrated. Larger cleft-like spaces, lined with endothelium and communicating with the lymphatic system, are also found surrounding the vessels, perivascular spaces. These are present in man in the Haversian canals. of bone tissue, around the vessels of the central nervous system, etc., and are separated from the actual vessel-wall by flattened endothelial cells. As in the case of the so-called perilymphatic spaces, the walls of the perivascular spaces are joined here and there by connective-tissue trabeculæ covered by endothelium. Such structures exist in the perilymphatic spaces of the ear, the subdural spaces of the pia, the subarachnoidal space, the lymph-sinuses, etc. The perivascular spaces are better developed in the lower animals (amphibia, reptilia, etc.) than in mammalia.

The cell-spaces, with their anastomosing processes, found in connective tissue and previously described as the lymph-canalicular system, possess no endothelial lining and communicate directly or indirectly with the lymph capillaries. To the lymphatic system. belong also the body-cavities, the pleural, pericardial, and peritoneal cavities. The walls of these consist of a connective-tissue stroma rich in lymph-spaces, lymph capillaries and lymph-vessels, and are lined by a layer of mesothelial cells. In them are found the stigmata and stomata mentioned in a former section. (See p. 85.) The synovial spaces belong also to the lymphatic system; they are lined by a layer of endothelial cells.

Mention has been made of the migration of leucocytes and, under certain conditions, of red blood-cells through the walls of blood capillaries, and in the case of the former through the walls of

lymph capillaries and lymph-vessels and spaces. This diapedesis of leucocytes probably takes place by a wandering of these cells through the stigmata and intercellular cement uniting the endothelial cells lining these spaces, and through the stomata in regions where these occur. According to later investigations, it would seem that leucocytes may bore through endothelial cells, and thus migrate from the vessel or space in which they are found previous to such migration. Kolossow (93), as a result of his investigations, advances still another theory. He believes that he has demonstrated that the cells lining the body-cavities are joined to each other by protoplasmic processes, and that their inner surfaces are covered by a cuticular membrane. These structures are especially well seen in the serous membranes of certain reptiles. Between the cells and between the protoplasmic processes connecting them are spaces which may be compared to the intercellular spaces found in the epidermis. It is thought by him that on stretching the serous membranes, the spaces between the lining cells become larger, and the cuticular portions of the cells become separated from each other, and in this way the stomata and stigmata are thought to be temporarily formed, and through these the migration of the leucocytes is believed to occur. This process is also supposed to occur in the smaller vessels and in the vascular and lymphatic capillaries. However, this whole question needs further investigation.

C. THE CAROTID GLAND (GLANDULA CAROTICA, GLOMUS CAROTICUM).

At the point where the common carotid divides, there lies in man a small oval structure about the size of a grain of wheat, known as the carotid gland or the glomus caroticum. It is imbedded in connective tissue, surrounded by many nerve-fibers, and on account of its great vascularity has a decidedly red color. The connectivetissue envelope of the gland penetrates into the interior in the form of septa, which divide its substance into small lobules, and these in turn into smaller round masses, the cell-balls. A small branch from the internal or external carotid enters the gland, where it branches, sending off twigs to the lobules, and these in turn still smaller divisions to the cell-balls. The latter vessels break up into capillaries, which merge at the periphery of each cell-ball to form a small vein, from which the larger trunks that pass from the lobules are derived. Each lobule is thus surrounded by a venous plexus from which the larger veins originate that leave the organ at several points. The cell-balls are composed of cellular cords, or trabeculæ, the elements of which are extremely sensitive to the action of reagents. The cells are round or irregularly polygonal and separated from each other by a scanty reticular connective tissue. The capillaries already mentioned come in direct contact

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