6.4: The Forelimb
The forelimb consists of: Humerus, radius and ulna, carpals, metacarpals, digits or phalanges (see diagram 6.6). The top of the humerus moves against (articulates with) the scapula at the shoulder joint. By changing the number, size and shape of the various bones, fore limbs have evolved to fit different ways of life. They have become wings for flying in birds and bats, flippers for swimming in whales, seals and porpoises, fast and efficient limbs for running in horses and arms and hands for holding and manipulating in primates (see diagram 6.8).
Diagram 6.6 - Forelimb of a dog
Diagram 6.7. Hindlimb of a dog
Diagram 6.8 - Various vertebrate limbs
Diagram 6.9 - Forelimb of a horse
In the horse and other equines, the third toe is the only toe remaining on the front and real limbs. Each toe is made up of a proximal phalange, a middle phalange, and distal phalange (and some small bones often referred to as sesamoids. In this image, the proximal phalange is labeled P3 and the distal phalange is labeled hoof. (which is more properly the name of the keratin covering that we see in the living animal).
The legs of the horse are highly adapted to give it great galloping speed over long distances. The bones of the lower leg and foot are greatly elongated and the hooves are actually the tips of the third fingers and toes, the other digits having been lost or reduced (see diagram 6.9).