Fat does not gather within the cranium, the lungs or the eyelids, where its accumulation would clog the organs.
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.