Skeletal
Skeletal, or striated, muscle tissues are attached to the bones and give shape to the body. They are responsible for allowing body movement. This type of muscle is sometimes referred to as striated because of the striped appearance of the muscle fibers under a microscope (fig. 1-9). They are also called voluntary muscles because they are under the control of our conscious will. These muscles can develop great power.

Smooth
Smooth, or nonstriated, muscle tissues are found in the walls of the stomach, intestines, urinary bladder, and blood vessels, as well as in the duct glands and in the skin. Under a microscope, the smooth muscle fiber lacks the striped appearance of other muscle tissue (fig. 1-10). This tissue is also called involuntary muscle because it is not under conscious control.

Cardiac
The cardiac muscle tissue forms the bulk of the walls and septa (or partitions) of the heart, as well as the origins of the large blood vessels. The fibers of the cardiac muscle differ from those of the skeletal and smooth muscles in that they are shorter and branch into a complicated network (fig. 1-11). The cardiac muscle has the most abundant blood supply of any muscle in the body, receiving twice the blood flow of the highly vascular skeletal muscles and far more than the smooth muscles. Cardiac muscles contract to pump blood out of the heart and through the cardiovascular system. Interference with the blood supply to the heart can result in a heart attack.

MAJOR SKELETAL MUSCLES
In the following section, the location, actions, origins, and insertions of some of the major skeletal muscles are covered. In figures 1-28 and 1-29 the superficial skeletal muscles are illustrated. Also note, the names of some of the muscles give you clues to their location, shape, and number of attachments.

Temporalis
The temporalis muscle is a fan-shaped muscle located on the side of the skull, above and in front of the ear. This muscle's fibers assist in raising the jaw and pass downward beneath the zygomatic arch to the mandible (fig. 1-29). The temporalis muscle's origin is the temporal bone. It is inserted in the coronoid process (a prominence of bone) of the mandible.

Masseter
The masseter muscle raises the mandible, or lower jaw, to close the mouth (fig. 1-28). It is the chewing muscle in the mastication of food. It originates in the zygomatic process and adjacent parts of the maxilla and is inserted in the mandible.

Sternocleidomastoid
The sternocleidomastoid muscles are located on both sides of the neck. Acting individually, these muscles rotate the head left or right (figs. 1-28 and 1-29). Acting together, they bend the head forward toward the chest. The sternocleidomastoid muscle originates in the sternum and clavicle and is inserted in the mastoid process of the temporal bone. When this muscle becomes damaged, the result is a common condition known as a "stiff neck."

Trapezius
The trapezius muscles are a broad, trapezium-shaped pair of muscles on the upper back, which raise or lower the shoulders (figs. 1-28 and 1-29). They cover approximately one-third of the back. They originate in a large area which includes the 12 thoracic vertebrae, the seventh cervical vertebra, and the occipital bone. They have their insertion in the clavicle and scapula.

Pectoralis Major
The pectoralis major is the large triangular muscle that forms the prominent chest muscle (fig. 1-28). It rotates the arm inward, pulls a raised arm down toward the chest, and draws the arm across the chest. It originates in the clavicle, sternum, and cartilages of the true ribs, and the external oblique muscle. Its insertion is in the greater tubercle of the humerus.

Deltoid
The deltoid muscle raises the arm and has its origin in the clavicle and the spine of the scapula (figs. 1-28 and 1-29). Its insertion is on the lateral side of the humerus. It fits like a cap over the shoulder and is a frequent site of intramuscular injections.

Biceps Brachii
The biceps brachii is the prominent muscle on the anterior surface of the upper arm (fig. 1-28). Its origin is in the outer edge of the glenoid cavity, and its insertion is in the tuberosity of the radius. This muscle rotates the forearm outward (supination) and, with the aid of the brachial muscle, flexes the forearm at the elbow.

Figure 1-28.-Anterior view of superficial skeletal muscles.