Chapter 4 Muscular system
The muscular system includes all the skeletal or striated muscles within the body. Striated muscle is that tissue attached to the skeleton that is under voluntary or conscious control (for microscopic structure see Ch. 2).
Muscle structure and function
Muscle tone
Anatomy of a muscle
A ‘classically’ shaped muscle (Fig. 4.2) has a thick, fleshy central part called the belly and tapers at each end – the head. Here, the connective tissue muscle sheath is continuous with the dense fibrous connective tissue of the tendon that attaches the muscle to a bone. A muscle is attached to a bone at two points: its starting point is called its origin; this moves least during contraction. The opposite end, where the muscle inserts on the bone, is called the insertion. However, a muscle can have more than one belly, all inserting at one point, in which case the muscle is said to have a number of heads, e.g. the biceps muscle has two heads. The length of the tendon attaching the muscle to a bone will vary, and in some cases tendons are far longer than the muscle itself, e.g. flexor and extensor tendons running over the digits.
The skeletal muscles of the body can be classed as either intrinsic or extrinsic:
The skeletal muscles
To simplify the study of these muscles we can look at one area of the body at a time.
Muscles of the head
Muscles of facial expression
The muscles of facial expression are intrinsic muscles that move the lips, cheeks, nostrils, eyelids and external ears. A number of muscles are responsible for these movements, all of which are innervated by the facial nerve (cranial nerve VII).
Muscles of mastication
The main muscles responsible for the masticatory or ‘chewing’ action of the jaw (Fig. 4.4) are:
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