Arm: Humerus, Radius and Ulna

The firs vertebrae lacked jaws. These animals, similar to lampreys and hagfish, also lacked paired fins. Jaws and fins evolved 400 million years ago, allowing fish to more effectively locomate and feed. Jawed fish have paired fins set on flat plates of bone that are attached to the muscles of their body walls. The paired fins, felxible fans of small bones, are used primilary as aid in stablizing and streeing. The limbs of terrestial animals evolved from this structural arrangement as fins were transformed into rod-bearing segments. Although the limbs of land vertebrates appear very diffrent from fish fins. the two homologous structres are actually highly comparable.
Each vertebrate has a base and 3 segments.
The base, The limb gridles = The old basal fin plates of fish, which evolved to take on the function of transferring the weight of the body to the terrestial tetrapod's limbs.
The proximal vertebrate limb segment constitute the upper arm and thight .
The intermediate limb segments, the forearm and foreleg, each bear 2 bone in humans, the radius and ulna in the upper limb, and their serial homologs, the tibia and fibula, in the leg.

Humerus
The upper arm bone. it is the largest bone of the upper limb. It comprises a proximal end wih a round, articular head, a shaft, and an irregular distal end. The humerus articulates proximally with the glenoid fossa of the scapula and distally with both the radius and the ulna.
Ossification
The humerus is ossified from eight centers, one for each of the following parts:
the body, the head, the greater tubercle, the lesser tubercle, the capitulum, the trochlea, and one for each epicondyle.
The center for the body appears near the middle of the bone in the eighth week of fetal life, and soon extends toward the extremities. At birth the humerus is ossified in nearly its whole length, only the extremities remaining cartilaginous. During the first year, sometimes before birth, ossification commences in the head of the bone, and during the third year the center for the greater tubercle, and during the fifth that for the lesser tubercle, make their appearance.
By the sixth year the centers for the head and tubercles have joined, so as to form a single large epiphysis, which fuses with the body about the twentieth year.
The lower end of the humerus is ossified as follows. At the end of the second year ossification begins in the capitulum, and extends medialward, to form the chief part of the articular end of the bone; the center for the medial part of the trochlea appears about the age of twelve. Ossification begins in the medial epicondyle about the fifth year, and in the lateral about the thirteenth or fourteenth year. About the sixteenth or seventeenth year, the lateral epicondyle and both portions of the articulating surface, having already joined, unite with the body, and at the eighteenth year the medial epicondyle becomes joined to it.