In early terrestrial tetrapods ( [link] ), the limbs were splayed out to the side, reflecting the position of the fins in their fishy ancestors. This resulted in a form of motion that was similar to performing push-ups while walking, which requires large muscles to move the limbs toward the midline. This is not an efficient form of locomotion, and selective pressures soon led to a configuration where the limbs were placed underneath the body, so that each stride requires less energy to move the animal forward. The rotation around the point of contact became a motion that is more like a pendulum when the limbs are underneath the body, producing a stride that was much more efficient for movement over land. Additional changes were required in the appendicular skeleton to accommodate the new ranges of motion that were enabled by that limb placement.
Muscles
Muscle cells are specialized for contraction. Muscles allow for motions such as walking, and they also facilitate bodily processes such as respiration and digestion. The body contains three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle ( [link] ).
Skeletal muscle tissue forms skeletal muscles, which attach to bones or skin and control locomotion and any movement that can be consciously controlled. Because it can be controlled by thought, skeletal muscle is also called voluntary muscle. Skeletal muscles are long and cylindrical in appearance; when viewed under a microscope, skeletal muscle tissue has a striped or striated appearance. The striations are caused by the regular arrangement of contractile proteins (actin and myosin). Actin is a globular contractile protein that interacts with myosin for muscle contraction. Skeletal muscle cells form by fusion of many muscle cells called myoblasts, and thus have multiple nuclei present in a single cell.
Smooth muscle tissue occurs in the walls of hollow organs such as the intestines, stomach, and urinary bladder, and around passages such as the respiratory tract and blood vessels. Smooth muscle has no striations, is not under voluntary control, has only one nucleus per cell, is tapered at both ends, and is also called involuntary muscle.
Cardiac muscle tissue is only found in the heart, and cardiac contractions pump blood throughout the body and maintain blood pressure. Like skeletal muscle, cardiac muscle is striated, but unlike skeletal muscle, cardiac muscle cannot be consciously controlled and is called involuntary muscle. It has one nucleus per cell, is branched, and is distinguished by the presence of intercalated disks. Intercalated disks are ion-permeable junctions between individual cardiac muscle cells which allow for synchronized contractions of the various regions of the heart.