NERVE TISSUE
Nerve tissue is the most complex tissue in the body. It is the substance of the brain, spinal cord, and nerves. Nerve tissue requires more oxygen and nutrients than any other body tissue. The basic cell of the nerve tissue is the neuron (fig. 1-12). This highly specialized cell receives stimuli from, and conducts impulses to, all parts of the body.

ORGANS

LEARNING OBJECTIVE: Recall how organs and body systems are composed of two or more kinds of tissue that perform specialized functions within the body.

As a group of similar cells forms tissues, two or more kinds of tissues grouped together and performing specialized functions constitute an organ. Organs are grouped together to form systems (such as the urinary system, composed of the kidneys, ureters, bladder, and urethra).

THE SKELETAL SYSTEM

LEARNING OBJECTIVE: Identify the parts of bone and their functions.

The skeleton, the bony framework of the body, is composed of 206 bones (fig. 1-13). It supports and gives shape to the body; protects vital organs; and provides sites of attachment for tendons, muscles, and ligaments. The skeletal bones are joined members that make muscle movement possible.

ANATOMY OF BONES
Osteology is the study of the structure of bone. Bone is made up of inorganic mineral salts (calcium and phosphorus being the most prevalent) and an organic substance called ossein. If human bones were soaked in dilute acid until all inorganic mineral salts were washed out, all that would remain would be a flexible piece of tissue that could be easily bent and twisted. Inorganic mineral salts give bone its strength and hardness.

Bone consists of a hard outer shell, called compact bone, and an inner spongy, porous portion, called cancellous tissue (fig. 1-14). In the center of the bone is the medullary canal, which contains marrow. There are two types of marrow, red and yellow. Yellow marrow is ordinary bone marrow in which fat cells predominate. It is found in the medullary canals and cancellous tissue of long bones. Red marrow is one of the manufacturing centers of red blood cells and is found in the articular ends of long bones and in cancellous tissue.

At the ends of the long bones is a smooth, glossy tissue that forms the joint surfaces. This tissue is called articular cartilage because it articulates (or joins) with, fits into, or moves in contact with similar surfaces of other bones. The thin outer membrane surrounding the bone is called the periosteum. An important function of the periosteum is to supply nourishment to the bone. Capillaries and blood vessels run through the periosteum and dip into the bone surface, supplying it with blood and nutrients. The

Figure 1-12.-Neuron.

Figure 1-11.-Cardiac muscle tissue.

periosteum is the pain center of the bone. When a bone fractures, the pain that is felt comes from the periosteum, not the bone proper. Periosteum also forms new bone. The diaphysis is the elongated, cylindrical portion (or "shaft") of the bone that is between the epiphyses (sing. epiphysis) or ends of the bone.