II. Reading and Comprehension

1. Read the text carefully to obtain detailed understanding of it.

A tissue is a group of cells that are similar in structure and function. Each tissue is a group of cells with its own characteristic intercellular substance often called the matrix. The human body has four main types of tissue: muscle, nervous, epithelial, and connective tissue. A number of tissues are, as a rule, associated with each other informing vous organs and organs systems of the body.

Muscle tissue is composed of cells that contract. According to the location, there arc three kinds of muscle tissue: skeletal muscle - attached to bones; visceral muscle - in the walls of hollow internal structures such as blood vessels, intestines, uterus and many other; cardiac muscle - composes the wall of the heart. Skeletal muscle tissue moves bones; cardiac muscle tissue, found in the heart, pumps blood through the body; smooth muscle tissue, found in many organs, moves substances through the body.

Nervous tissue consists of cells that transmit messages through the body. These cells, with the corresponding accessory structures distributed over the body or near its surface, produce various sensory systems: the exteroceptive system which is concerned with the reception of impulses from the surface of the body, the interoceptive from the internal organs, and the proprioceptive from muscles, joints and tendons. Other nerve cells become connected with the peripheral effector organs, as muscles and glands, forming the neuromotor and secretory systems. Still other nerve cells, mostly collected in a large, central mass, play the role of mediators or integrators. Their function is to receive, select, combine, distribute, modify the excitations arriving from the receptive surfaces or from the inner organs, and finally to influence properly the peripheral effectors. These cells form the so-called central nervous system. Nerve cells respond to changes occurring in all parts of the body. The brain, spinal cord, sensory organs, and nerves are composed of nervous tissue. The size, shape and other peculiarities of the body vary in the extreme, producing countless varieties or types of nerve cells. The nervous system is characterized by the multiplicity of cellular forms and intercellular connections and by the complexity of its functioning. This multiplicity and complexity is the chief feature that distinguishes the nervous system from the other tissues.

Figure 1. Diagram of the human body tissues

Epithelial tissue protects, secretes, and absorbs. The height of epithelial cells varies from a very flat type (squamous) to a tall rod-like (columnar) cell. Forms intermediate between these two types are cuboidal. An epithelial layer may be one or many cells in thickness. When it is composed of a single layer, it is called a simple epithelium; when two or more cells in thickness, it is stratified.

Epithelial cells bind tightly together, forming a solid sheet that covers, protects, or lines a body part. Examples of epithelial cells include the skin and the cells that line the organs of the body cavity. The cells in each type of epithelial tissue arc shaped and arranged so as to perform a particular function. For example, the thin, flat cells that line the inside of the mouth are arranged like tiles on a floor. These cells shield the cells that lie under them.

Connective tissues help support the body. They include bone, cartilage, tendons, fat, blood, and lymph. Unlike epithelium, which consists almost entirely of cells, the various connective tissues arc all characterized by the enormous development of the intercellular substance. The cells are isolated, separated from their neighbours by relatively large areas of intercellular substance, the matrix. The matrix can be solid, semisolid, and in some cases. liquid. Bone cells arc surrounded by a hard, crystalline matrix due to the deposition of calcium salts in the matrix. A fibrous matrix that is less solid than bone surrounds cartilage cells, tendon cells, and fat cells. Various blood cells are suspended in a liquid matrix. The connective tissues are richly supplied with nerves which end in the tissue itself or go to epithelium and muscle. They are the connecting and supporting tissues of the body. The connective tissues affect a union of structures, permitting easy displacement of the connected parts. Where greater firmness is needed we have the tough fibrous membranes and bands, or the rigid segments of the skeleton. Tendon, ligament, cartilage and bone form together the more static portions of the locomotor apparatus.

Marrow is a soft tissue, which occupies the medullary cavity of the long bones and all the spaces between the trabeculae of the spongy bone. It consists of a delicate reticular connective tissue where various kinds of cells lie. Two varieties of marrow are recognized: red and yellow.

Red marrow. Red marrow is the only type found in young bones, but in the adult it is restricted to the vertebrae, sternum, ribs, cranial bones and the epiphyses of long bones. It is the chief blood forming organ of the adult body, being the sole normal source of the red blood cells and granular leucocytes.

Yellow marrow. Yellow marrow consists in the main of fat cells which have gradually replaced the other marrow elements. Under certain conditions the yellow marrow loses most of its fat and assumes a reddish colour and gelatinous consistency. It is then known as "gelatinous marrow".

With an adequate stimulus, such as fracture of the bone, yellow marrow may reassume the character of red marrow and play an active part in the process of bone repair.