In this article we will discuss about the muscular system of toad. This will also help you to draw the structure and diagram of skeletal muscles of toad.
It has already been pointed out that the muscular system is composed of muscles which are used for movements.
There are two kinds of muscles:
(i) The voluntary muscles, which can be contracted at will and are attached to the skeleton; and
(ii) The involuntary muscles, which are not controlled by desire and occur as sheets on the wall of organs such as heart, blood vessels, digestive canal, etc.
The main mass of external musculature of toad is formed by the voluntary muscles. They are attached to the bones and are hence designated as skeletal muscles. There are about two hundred skeletal muscles in the body of a toad and the more important ones are shown in the adjoining diagram (Fig. 21). Each is composed of bundles of muscle fibres bound together by connective tissue.
Some muscles occur as sheets, such as the obliquus externus covering the belly. Others are elongated and spindle- shaped, such as the deltoid and the gastrocnemius. A few have fibres arranged circularly round an opening; these are the sphincter muscles.
A muscle has at least two points of attachment fastened to separate bones. The end of the muscle by which it is attached to a relatively fixed bone is known as its origin. The other end, attached to a more movable bone, is its insertion. A muscle may or may not be connected to a fibrous tendon at its ends. When both the ends are tendinous, the fleshy central part of a muscle is distinguished as the belly.
Muscles possess the power of contraction and relaxation. When a muscle contracts, its two ends are opposed and the bone to which it is inserted is brought nearer to the bone from which it originates. It is in this way that movements are effected. Each muscle is supplied by a motor nerve, the stimulation of which is followed by the contraction of the muscle.
According to their mode of action, the muscles may be of the following types:
(a) An extensor straightens or extends a part; Triceps muscle straightens the forearm.
(b) A flexor bends one part upon another; Biceps bends forearm upon the arm.
(c) An adductor draws a limb towards the axis of the body; the latissimus dorsi adducts the forelimb.
(d) An abductor draws a limb away from the axis of the body; the deltoid abducts the forelimb.
(e) A levator raises a part; the masseter raises the lower jaw to close the mouth.
(f) A depressor lowers a part; the mylohyoid lowers the floor of the buccal cavity.
(g) A rotator rotates a part; the small pyriformis rotates the hind limb in the acetabulum.
Muscles are usually arranged in opposing groups, so that when one set is contracting the opposite set is kept in a relaxed state. To be more precise, when biceps, which is a flexor muscle, is contracting, the triceps, which is an extensor muscle, is kept relaxed. This co-ordination is effected through the agency of the nervous system.