Skeletal Muscle Groups
The study of muscles is made simpler by grouping them according to body regions. Knowing how muscles are named can also help in remembering them. A number of different characteristics are used in naming muscles, including the following:
* Location, named for a nearby bone, for example, or for position, such as lateral, medial, internal, or external;
* Size, using terms such as maximus, major, minor, longus, brevis;
* Shape, such as circular (orbicularis), triangular (deltoid), trapezoid (trapezius);
* Direction of fibers, including straight (rectus) or angled (oblique);
* Number of heads (attachment points) as indicated by the suffix -ceps, as in biceps, triceps, quadriceps;
* Action, as in flexor, extensor, adductor, abductor, levator.
Often, more than one feature is used in naming. Refer to Figures 4-9 and 4-10 as you study the locations and functions of some of the skeletal muscles and try to figure out why each has the name that it does. Although they are described in the singular, most of the muscles are present on both sides of the body.
Figure 4-9 Superficial muscles, anterior view. Associated structure is labeled in parentheses.
Figure 4-10 Superficial muscles, posterior view. Associated structures are labeled in parentheses.
Muscles of the Head
The principal muscles of the head are those of facial expression and of mastication (chewing) (Fig. 4-11). The muscles of facial expression include ring-shaped ones around the eyes and the lips, called the orbicularis muscles because of their shape (think of “orbit”). The muscle surrounding each eye is called the orbicularis oculi, whereas the muscle of the lips is the orbicularis oris. These muscles, of course, all have antagonists. For example, the levator palpebrae superioris, or lifter of the upper eyelid, is he antagonist for the orbicularis oculi. ne of the largest muscles of expression forms the leshy part of the cheek and is called the buccinator. Used in whistling or blowing, it is sometimes eferred to as the trumpeter’s muscle. You can readily hink of other muscles of facial expression: for instance, he antagonists of the orbicularis oris can produce smile, a sneer, or a grimace. There are a number of scalp uscles by means of which the eyebrows are lifted or rawn together into a frown. There are four pairs of muscles of mastication, all of which insert on and move the mandible. The largest are the temporalis, which is superior to the ear, and the masseter at the angle of the jaw. The tongue has two groups of muscles. The first group, called the intrinsic muscles, is located entirely within the tongue. The second group, the extrinsic muscles, originates outside the tongue. It is because of these many muscles that the tongue has such remarkable flexibility and can perform so many different functions. Consider the intricate tongue motions involved in speaking, chewing, and swallowing. Figure 4-11 shows some additional muscles of the face.
Figure 4-11 Muscles of the head. Associated structure is labeled in parentheses.
Muscles of the Neck
The neck muscles tend to be ribbonlike and extend up and down or obliquely in several layers and in a complex manner (Fig. 4-11). The one you will hear of most frequently is the sternocleidomastoid, sometimes referred to simply as the sternomastoid. This strong muscle extends superiorly from the sternum across the side of the neck to the mastoid process. When the left and right muscles work together, they bring the head forward on the chest (flexion). Working alone, each muscle tilts and rotates the head so as to orient the face toward the side opposite that muscle. If the head is abnormally fixed in this position, the person is said to have torticollis, or wryneck; this condition may be due to injury or spasm of the muscle. A portion of the trapezius muscle (described later) is located at the posterior of the neck, where it helps hold the head up (extension). Other larger deep muscles are the chief extensors of the head and neck.