The branchial arch skeleton is derived from the embryonic counterparts of the gill arches that support the mouth and pharynx of present-day adult fish and tailed amphibians. The most primitive skeletal rudiments of the branchial arches develop from neural crest cells that migrate into the arches, not from the mesoderm of the arches. The neural crest rudiments become cartilaginous and are retained as cartilage in present-day adult cartilaginous fish, such as the shark, to support the jaw and aqueous respiratory system. In the evolutionary transformation from water breathing to air breathing, much of the skeleton of the aqueous respiratory system was modified to become parts of the air respiratory system, as well as of the modified acoustic apparatus. The human embryo goes through the essential structural stages of this evolutionary waterbreathing to air-breathing transformation. Some of the cartilages remain in the adult human (laryngeal cartilages), whereas others become bone (hyoid, styloid process, and ossicles of the middle ear). The branchial arch components originally subserved the function of mastication as well as that of respiration. Although the primitive cartilages of the first branchial arches become the skeletons of the upper and lower jaws in cartilaginous fish, they do not do so in humans, in whom the maxillae and mandible are derived from membrane bones.

Because the brain grows large before birth, the calvaria is much larger than the facial skeleton in the neonate with a ratio of 8 : 1, compared with a ratio of 2 : 1 in the adult (see Plate 1-7).

APPENDICULAR SKELETON

The appendicular skeleton consists of the pectoral and pelvic girdles and the bones of the free appendages attached to them. The paired appendages of land vertebrates evolved from the paired fins of fish. The development of the human limbs is a résumé of their evolution.

The upper limb buds appear first, differentiate sooner, and attain their final relative size earlier than the lower limbs (see Plates 1-3, 1-8, and 1-9). Not until birth do the lower limbs equal the upper limbs in length (see Plate 1-7). However, throughout childhood, the lower limbs elongate faster than the upper limbs. In essence, an upper limb was never a lower limb, and vice versa; each has its own unique evolutionary and developmental history. Even so, it is interesting that the structures of the mature upper and lower limbs have a number of similarities. They are most similar during the earliest stages of development, when both sets of finlike appendages point caudally. They then become paddle-like and project outward almost at right angles to the body wall. After this, they bend at the elbow and knee directly anteriorly, so that the elbow and knee point laterally, or outward, and the palm and sole face the trunk. Then a series of major changes occurs that causes the upper and lower limbs to differ markedly both structurally and functionally (see Plate 1-8). By the seventh week, both undergo a 90-degree torsion about their long axes, but in opposite directions, so that the elbow points caudally and the knee points cranially. Accompanying this torsion is a permanent twisting of the entire lower limb, which results in its cutaneous innervation assuming a twisted, “barber pole” arrangement (see Plate 1-9). This would be similar to twisting the upper limb so that the forearm and hand become fully and permanently pronated.