Cardiac muscles are striated but involutary. They have branching strands and come from mesenchyme. Muscles can only contract. Relaxation is the lack of contraction. They often work in pairs called agonists and antagonists .
Some examples are abductors and aductors, flexors and extensors, constrictors and dilators.
Cut through the skin of the cat or rat from the pelvic region to the sternum and the limbs. Skin this area by inserting the handle of the scalpel between the skin and the muscles. The muscles are connected to the skin by faschia and muscles attached to the faschia may twitch the skin. Under the skin you can see the abdominal muscles. The white line mid-ventrally is the linea alba formed by a muscle to muscle attach ment of faschia called an aponeurosis .