PHYSICAL PROPERTIES OF THE BODY.
The Tissues.-- The solid organized substances of which the human body is composed, are called tissues. There are various kinds of tissues.
The Cellular Tissue, commonly called areolar, is made up of small fibers and bands woven together into a sort of network, with numerous little spaces opening into each other. These spaces are filled with a watery fluid; and when this is greatly increased by disease, so as to cause the parts to swell, and the skin to shine, the person has anasarca, or celldropsy. The uses of this tissue are to give parts and organs a kind of elastic cushion to rest upon, so that they may not be bruised and injured by the shocks of life; to make a kind of safe highway for delicate vessels to pass from one part of the body to another; and to furnish a beautifully arranged lodgment for the watery fluid which gives such roundness, smoothness, and grace to the human form. The opening of the cells into each other explains the reason why feeble persons have swelled feet and ankles in the evening, and not in the morning the fluid settling down from cell to cell, into the lowest parts, while they are up during the day, and running back to its proper place while they are lying down during the night. The Mucous Tissue, or mucous membrane, lines all the cavities which communicate with the air, as the mouth, stomach, bowels, lungs, etc. It is supplied with numerous small glands which secrete a sticky kind of fluid called mucus, to protect the surface from any injury which might be inflicted by air, or by irritating substances suspended in it.
The Serous Tissue, or membrane, lines all the cavities which do not communicate with the air, that is, all those which are shut, and have no outward opening. The skull, the chest, and the belly are lined by this kind of membrane. The membrane itself forms a closed sac, one layer of it being attached to the cavity it lines, while the other is folded back upon and around the contents of the cavity, which are left outside of the sac. A watery fluid oozes from the inner surface of the sac, to make its sides glide easily upon each other. When some disease causes this water to be poured out too freely, so as to fill or partly fill the cavity, we have dropsy of the brain, or chest, or abdomen, as the case may be.
The Dermoid Tissue covers the whole outside of the body. We call it the 8kin, or cutis. It is similiar in structure to the mucous membranes, which are a mere continuation of it. It is harder than the mucous membrane, because more exposed to injury. In health, it never ceases to secrete and throw off a fluid which we call insensible perspiration while it is in the form of an in~isible vapor, and perspiration, or sweat, when it is so increased as to be seen. So great is the sympathy between this dermoid covering of the body and the mucous membranes, that when it is chilled so as to stop the invisible perspiration, the internal membrane becomes affected, and we have a sore throat, or diarrhoea, or running at the nose; that is to say, when the skin cannot sweat, the mucous membrane begins to sweat.
The Fibrous Tissue consists of closely united fibres, and for whatever purpose used, forms a fin6, dense, and enduring body. In some cases it takes the form of a membrane, as the dura mater, which lines the interior of the skull and spinal column. The ligaments which hold the bones together, and the tendons or cords, which fasten the muscles to the bones, are fibrous bodies. It is this firm substance of which rheumatism frequently takes hold, and this is the reason why it lingers so much about the joints. It sometimes takes hold of the ligament which fastens the deltoid muscle to the bone of the upper arin, about twothirds of the way from the elbow to the shoulder. This muscle lifts up the arm. In this form of rheumatism, therefore, the arm hangs helpless at the side.
The Cartilaginous Tissue covers the ends of the bones where they come together to make a joint. It is well fitted to make the joint work easy, being smooth, hard, and elastic.
The Osseous or Bony Tissue varies in its composition, density, and strength, according to the age of the person, and the uses of the bone.
The Muscular Tissue, or muscle, being made for a great deal of pulling and lifting, is formed something like a, rope, except that there is no twisting. Many small fibres or filaments unite to form fasciculi. A fasciculus is a bundle of fibres surrounded by a delicate layer of cehtissue called 8arcolemma, just as a cord is a number of smaller threads of cotton or hemp bound together. A number of these fasciculi united together make a muscle, just as several cords, called strands, twisted together, make a rope. Figure I gives us a good view of the fibres and bundles,highly magnified.
The Adipose Tissue is the material which the human body works up into pots and cells containing fat. It is found chiefly under the skin and muscles of the belly, and around the heart and kidneys. By the increase of this tissue, persons may become enormously enlarged without having their muscles at all increased in size. Such a condition is to be deplored, the body having become merely the storehouse or depot of myriads of pots of fat.
The Nervous Tissue is composed of two
distinct kinds of matter, the one gray and pulpy, called eineritious,
the other white and fibrous, called medullary. The external part of the
brain and the internal portion of the spinal cord are composed of the
gray or ash colored tissue; the nerves are made only of the white or
fibrous matter, and are enclosed in a delicate sheath called neurilemma.
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