nutrients than any other body tissue. The basic cell of the nervous tissue is the neuron (fig. 3-11). This highly specialized cell receives stimuli from, and conducts impulses to, all parts of the body.
As a group of similar cells form tissues, similar tissues form organs such as the heart, liver, and kidneys. These organs are grouped together to form systems, such as the urinary system that is composed of the kidneys, ureters, bladder, and urethra.
Figure 3-12.Human skeleton.
The skeleton is the bony framework of the body, composed of 206 bones (fig. 3-12). 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.
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. When human bone is soaked in dilute acid until all inorganic mineral salts are washed out, all that remains is a flexible piece of tissue that can easily be bent and twisted. The inorganic mineral salts give bone its strength and hardness.
Bone consists of a hard outer shell, called compact tissue, and an inner spongy, porous portion, called cancellous tissue (fig. 3-13). 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 (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 nourishment. The periosteum