Architecture And Shape Of Skeletal Muscles
The architecture and shape of a skeletal muscle depend on the arrangement of its fibers.
FORM, FEATURES, AND NAMING OF MUSCLES
All skeletal muscles, commonly referred to simply as “muscles,” have fleshy, reddish, contractile portions (one or more heads or bellies) composed of skeletal striated muscle. Some muscles are fleshy throughout, but most also have white noncontractile portions (tendons), composed mainly of organized collagen bundles, that provide a means of attachment.
The architecture and shape of muscles vary. The tendons of some muscles form flat sheets, or aponeuroses , that anchor the muscle to the skeleton (usually a ridge or a series of spinous processes) and/or to deep fascia (such as the latissimus dorsi muscle of the back) or to the aponeurosis of another muscle (such as the oblique muscles of the anterolateral abdominal wall). Most muscles are named on the basis of their function or the bones to which they are attached. The abductor digiti minimi muscle, for example, abducts the little finger. The sternocleidomastoid muscle (G. kleidos , bolt or bar, clavicle) attaches inferiorly to the sternum and clavicle and superiorly to the mastoid process of the temporal bone of the cranium. Other muscles are named on the basis of their position (medial, lateral, anterior, posterior) or length (brevis, short; longus, long). Muscles may be described or classified according to their shape, for which a muscle may also be named:
- Flat muscles have parallel fibers often with an aponeurosis—for example, the external oblique (broad flat muscle). The sartorius is a narrow flat muscle with parallel fibers.
- Pennate muscles are feather-like (L. pennatus , feather) in the arrangement of their fascicles and may be unipennate, bipennate , or multipennate —for example, extensor digitorum longus (unipennate), rectus femoris (bipennate), and deltoid (multipennate).
- Fusiform muscles are spindle shaped with a round, thick belly (or bellies) and tapered ends—for example, biceps brachii.
- Convergent muscles arise from a broad area and converge to form a single tendon—for example, pectoralis major.
- Quadrate muscles have four equal sides (L. quadratus , square)—for example, the rectus abdominis, between its tendinous intersections.
- Circular or sphincteral muscles surround a body opening or orifice, constricting it when contracted—for example, orbicularis oculi (closes the eyelids).
- Multiheaded or multibellied muscles have more than one head of attachment or more than one contractile belly, respectively. Biceps muscles have two heads of attachment (e.g., biceps brachii), triceps muscles have three heads (e.g., triceps brachii), and the digastric and gastrocnemius muscles have two bellies. (Those of the former are arranged in tandem; those of the latter lie parallel.)
Source: Clinically Oriented Anatomy
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