- The second compares the number of all major postcranial elements, the femur, tibia, humerus, radius and ulna, with not just the mandible and maxilla but the isolated molars as well.
- With further outgrowth the radius and ulna develop within the inner mass of cells, then the wrist, and finally the hand with its digits.
- Thus, if the ridge is removed from a very early bud only the humerus will develop; but if it is removed at a later stage the radius and ulna and wrist will develop but the digits will be absent.
- Then, if cartilage is made only where there are peaks, a basic pattern will be established which could then be modified by positional information to give the characteristic patterns of the humerus, radius and ulna, wrist, and hand.
- Only when the cells leave the zone do some begin to differentiate into cartilage; and, as just stated, the cartilage elements are laid down in a proximo-distal sequence - first humerus, then radius and ulna, and only then wrist, and finally hand.
- The basic pattern consists of one main element, the humerus, followed by two smaller elements, the radius and ulna, then some small structures in the wrist, and finally the digits.
- There may be a collecting bias against the foot bones, however, because they are small and easily overlooked, and so a comparison has been made here between the proximal and distal limb elements, the former being the femur and humerus, and the latter represented by the tibia and radius (values for ulna are very similar).
- However, many vertebrate forelimbs have a similar basic structure - a single element, the humerus, followed by two elements, the radius and the ulna, and then three or more elements in the wrist and digits.
- All are diurnal raptors, and their skeletal element proportions have five distinct peaks corresponding to the mandible, humerus, ulna, femur-tibia, and isolated teeth.
- Not only is the pattern of cartilage elements in the early limb, such as the humerus, radius and ulna, and digits, quite simple, but the embryonic chick limb bud is quite easily accessible to experimental manipulation and is an excellent model for the human.
- The other skeletal elements displayed are mandible (mand), maxilla (max), scapula (scap), humerus (hum), radius (rad), ulna, pelvis (pelv), femur (fem), tibia (tib) and vertebrae (vert).
- Articular ends of limb bones have been partly eaten away, resulting in local damage very different from the corrosive effects of digestion, and processes such as the olecranon process of the ulna or the medial epicondyle of the humerus are often totally destroyed.
- (a) - (f) femur fragments; (g) - (l) tibia fragments; (m) - (s) humerus fragments; (t) - (w) ulna fragments.
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