CHAPTER VIII. THE VASCULAR AND LYMPHATIC SYSTEMS. FROM THE VASCULAR SYSTEM. THE HEART. ROM without, inwards the wall of the heart consists of the following layers : 1.-The Pericardium; 2.-The Myocardium; 3.-The Endocardium. 1.-The Pericardium.—The heart, like most of the other viscera contained in the cavity of the thorax or abdomen, is invested with a serous sac composed of two layers; a parietal, applied to the walls of the pleural mediastina, and a visceral layer covering the organ itself. The two are continuous with each other upon the roots of the various vessels which enter and leave the heart. The visceral pericardium, sometimes termed the 'epicardium '—which, by its epithelial surface, affords the smooth glistening appearance to the exposed heart, when the chest wall and parietal layer are opened-consists of an external layer of flat, polygonal, squamous cells with an internal fibrous layer beneath it, constituted for the most part of white fibrous tissue, with a considerable amount of the elastic element amongst it in the form of fibres. This layer contains capillaries, lymphatics and nerves, and is continuous with the connective tissue between the muscular fibres of the myocardium beneath it. 2. The Myocardium consists of a network of muscular fibres, the nature of which has already been described under the heading of cardiac muscle (page 187). Between the fibres is a certain amount of delicate, connective tissue, continuous on the one hand with the pericardium, and on the other with the endocardium, and containing blood capillaries, lymphatic spaces and nerves. 8.-The Endocardium resembles the pericardium in structure, but is much thinner. It consists of an inner endothelial lining with a layer of sub-endothelial connective tissue, continuous with that supporting the muscular substance of the myocardium. The endocardium differs from the intima of an artery in containing capillary blood-vessels and lymphatics. The epithelial lining also, like that of the pericardium, consists of polygonal instead of fusiform squames. The Valves of the Heart.-The auriculo-ventricular valves of the heart and the semi-lunar valves of the aorta and pulmonary artery may be regarded, structurally, as reduplications of the lining membrane, or endocardium. They are composed of fibrous tissue, for the most part of the white variety, with a layer of flattened epithelial cells on each surface. The valves are supplied with capillary blood-vessels like the rest of the endocardium. In the case of the semilunar valves, however, these only extend a little way into the thicker attached portion of the membrane, leaving the rest extra-vascular. THE BLOOD-VESSELS. Structure of a Medium-Sized Artery.-(Figs. 60 and 62).-One of the smaller arteries, such as the radial, cerebral, or lingual, has the following structure :— It consists of three coats; of which the internal, or intima, presents a smooth surface to the blood stream, and is also elastic, if the internal elastic lamina is considered part of it, as it usually is; the middle coat, or media, is contractile and elastic; and the external coat, or adventitia, is elastic, and also bears the blood-vessels (vasa vasorum) and nerves for the vessel itself. 1.-The Intima consists of these parts: (a,) An endotheliai lining of flattened elongated cells, with a somewhat sinuous outline, cemented one to another by their margins, their long axis parallel with that of the vessel they line; (b) Sub-endothelial connective tissue. Beneath the epithelium is a layer of connective tissue, which varies in nature and extent according to the size of the artery in question. In such as those mentioned above, this consists of a non-vascular, narrow layer of ordinary connective tissue, bounded on the out side with a tubular homogeneous elastic membrane-the internal elastic lamina. This is frequently of the nature of a fenestrated membrane of felted elastic fibres, but it may be a complete membrane, and show the merest trace of its fibrous origin. The smaller the artery, the less of the sub-endothelial connective tissue is there, until, ultimately, the epithelium lies directly upon the internal elastic lamina, which, in that case, occupies the position of a basement membrane. In a transverse section of a medium sized artery, such as we see in ordinary specimens of prepared tissues, the internal elastic lamina appears as a highly refractile sinuous band, surrounding the lumen of the vessel, bearing on its surface the rest of the intima, which may be represented by epithelium alone, or by epithelium with a layer of white fibrous tissue beneath it. The appearance of sinuosity, which the internal elastic lamina presents in section, is due to the fluting the tube undergoes, owing to the diminution in the lumen of the vessel, when the pressure of the fluid within it is removed. 2.-The Media, or middle coat of the artery, is the broadest of the three, and is composed of circularly arranged, non-striped muscle fibres, amongst the layers of which are found a varying number of elastic plates, and a small amount of delicate white fibrous tissue. The plates vary in proportion to the muscular substance, according to the size of the artery, i.e., to the more special function it is required to subserve. In the case of the aorta, especially, and the larger arteries as well, the function of the vessel is to a much greater extent of an elastic nature than in the case of the smaller vessels, such as the lingual or radial, in which it is more muscular; there may even be a preponderance of elastic and fibrous tissue over the muscular element in the media of the former, whereas in the latter, the muscular element is always in excess. The elastic plates are structurally of the same nature as the internal elastic lamina; that is to say, they are fenestrated membranes or felt-works of elastic fibres. In transverse section, they appear as highly refractile sinuous lines amongst the non-striped muscle fibres-the sinuosity, as in the case of the internal elastic lamina, being due to the longitudinal fluting of the plate. 3. The Adventitia, or external coat, consists of white fibrous tissue, with a network of elastic fibres running for the most part in a longitudinal direction. In this coat the vasa vasorum, or |