=Mucous Membranes.=—Mucous membranes line all the open cavities of the body, or all those cavities which communicate with the outside. At the edges of the openings into the body, the skin seems to stop and give place to a tissue which is redder, more sensitive, more liable to bleed, and is moistened by a fluid or mucous, as it is called. Really, however, the skin does not cease, but passes into a more delicate covering of the same general structure, and it is to this that the name mucous membrane is applied. The entire alimentary canal, the entire respiratory tract, and the genito-urinary tract, are lined with a mucous membrane. Mucous membrane secretes a mucous fluid. =Serous Membranes.=—Serous membranes line the closed cavities of the body. The pleurae, the pericardium and the peritoneum are examples of serous membranes. Serous membranes secrete a serous fluid. =Synovial Membranes.=—Synovial membranes are serous in character, and consist of loose connective tissue, containing fat, vessels and nerves, its inner surface being usually lined with secreting cells. The fluid secreted is yellowish-white or slightly reddish, resembling very much the white of an egg. It contains fats, salts, albumen, extractives from the lymph, and a fluid known as synovia. The chief function of this fluid is to act as an oil to lubricate the joints and surfaces in which there is any friction. Synovial membranes are divided into three classes, known as articular, bursal and vaginal. _Articular synovial membranes_ are found in every free movable joint. _Bursal synovial membranes_ are sacs interposed between the surfaces which move upon each other, producing friction, as in the gliding of a tendon or of the integument over projecting bony surfaces. _Vaginal synovial membranes_ serve to facilitate the gliding of a tendon in the bony canal through which it passes. [Illustration: FIG. 8—Section of artery. (Grunstein)] =Arteries.=—The arteries are cylindrical vessels which serve to convey the blood from both ventricles of the heart to every part of the body. They are called arteries from the Greek words which mean “to contain air,” and they were supposed, by our ancients, to have this function until the time of Galen, when he refuted this opinion and showed that these vessels, though for the most part empty after death, actually contained blood. The distribution of the arteries may be compared to a tree, the common trunk of which corresponds to the aorta, and the smallest twigs corresponding to the minute capillaries. When one artery communicates with another it is said to anastomose, and this communication is very free between the larger as between the smaller branches. Anastomosis between trunks of equal size is found where great activity of the circulation is requisite, as at the base of the brain, where the two vertebrals unite to form the basilar artery.