The separation of the skeletal system into axial and appendicular divisions provides a useful guideline for subdividing the muscular system as well:

1. The axial musculature arises on the axial skeleton. It positions the head and spinal column and also moves the rib cage, assisting in the movements that make breathing possible. It does not play a role in movement or support of either the pectoral or pelvic girdle or the limbs. This category encompasses roughly 60 percent of the skeletal muscles in the body.
2. The appendicular musculature stabilizes or moves components of the appendicular skeleton and includes the remaining 40 percent of all skeletal muscles.

Figure 11-3 provides an overview of the major axial and appendicular muscles of the human body. These are the superficial muscles, which tend to be relatively large. The superficial muscles cover deeper, smaller muscles that cannot be seen unless the overlying muscles are either removed or reflected--that is, cut and pulled out of the way. Later figures that show deep muscles in specific regions will indicate whether superficial muscles have been removed or reflected for the sake of clarity.

Paying attention to patterns of origin, insertion, and action, we will now examine representatives of both muscular divisions. This discussion assumes that you already understand skeletal anatomy. As you examine the figures in this chapter, you will find that some bony and cartilaginous landmarks are labeled for orientation purposes. These labels are shown in italics, to differentiate them from the muscles that are the primary focus of each figure. Should you need further review of skeletal anatomy, figure captions in this chapter indicate the relevant figure numbers in Chapters 7, 8, or 9.

The muscle tables that follow also contain information about the innervation of the individual muscles. Innervation is the distribution of nerves to a region or organ; the tables indicate the nerves that control each muscle. Many of the muscles of the head and neck are innervated by cranial nerves, such as the facial nerve, or seventh cranial nerve (N VII), which innervates the facial musculature. Cranial nerves originate at the brain and pass through the foramina of the skull. Spinal nerves are connected to the spinal cord and pass through the intervertebral foramina. For example, spinal nerve L₁ passes between vertebrae L₁ and L₂ . Spinal nerves may form a complex network, or plexus; one branch, such as the sciatic nerve of the thigh, may contain axons from several spinal nerves. Thus, many tables include the spinal nerves as well as the names of the peripheral nerves.

The gracilis muscle is attached to the anterior surface of the tibia at one end and to the pubis and ischium of the pelvis at the other. When the muscle contracts, hip flexion occurs. Which attachment point is the muscle's origin?

Muscle A abducts the humerus, and muscle B adducts the humerus. What is the relationship between these two muscles?

What does the name flexor carpi radialis longus tell you about this muscle?

FIGURE 11-3 An Overview of the Major Skeletal Muscles. (a) An anterior view. (b) A posterior view.