THE APPENDICULAR SKELETON. Appendages in vertebrates may be divided into two kinds: (1) paired, and (2) unpaired or median. Paired appendages are represented by the lateral fins of fishes, and the legs and wings of higher animals. Unpaired appendages are confined chiefly to fishes, and occur in the sagittal plane dorsally, posteriorly, and ventrally. Certain deep-lying structures which support the appendages must be considered in connection with them. The origin of the appendages is a much disputed question. Two views, however, have gained cur rency. That of Gegenbaur depends wholly upon anatomical evidences; that of Balfour, Dohrn, and others is based wholly upon embryological evidence. Gegenhaur's theory is that the shoul der and pelvic girdles have each been derived from one gill-arch and that the appendages are modi fied gill-rays—the bony processes of the gill arches, supporting the gill-membrane. Now in such a gill-arch one frequently finds one of the middle gill-rays much more highly developed than the others. Sometimes on this larger ray lateral rays arrange themselves. From this latter con dition, which occurs in Ceratodus, may be de rived and explained the skeleton of the limbs of fishes and of all the higher vertebrates. The girdles have been derived from gill-arches. The theory of Dohrn rests almost wholly upon the evidence afforded by ontogenetic development. The muscles which enter the arm are not derived from one mesodermal somite, but from a number (ten to thirty), and as each gill-areh corre sponds to one metamere„ the appendages cannot be derived from gill-arches and their rays. More over, the muscles of the are derived from the dorsal muscle-plates those of the branchial arches from the lateral plates of the bead, hence the musculature of the two are de rived from entirely different sources. Dohrn be lieves the limbs have arisen front a continuous fin, which is paired anteriorly, but fuses poste riorly to form an unpaired ventral fin that ex tends up over the tail to the mid-dorsal linc. By a failure of the development of a part of this continuous fin two paired ventral fins appear, as well as median or unpaired ventral. caudal, and dorsal fins. The evidence for this Dohrn finds in the fact that masses of muscles are constricted off from the muscle-plate in the interappendicular region just as at the appendages; these muscles later degenerate. Dohrn also finds muscle-masses given off in each somite to the median fin. hence the median tin is to be regarded as derived from two fused lateral fins.
The paired appendages of vertebrates fall into two types: ( ) that of fishes, and (t) that of higher vertebrates. We may distinguish in each case two parts: an axial, the girdle, and a peri pheral, the free appendage. It seems probable
that the free appendage was developed first, and that the girdle arose from the necessity of a firm er axial support for them. The skeleton of the fins of fishes is composed of bone, whereas in selachians it is cartilaginous. The plan of the formation of the anterior and posterior append ages of higher vertebrates is the same, and the re markable correspondence of their anterior and posterior limbs is to be accounted for by force of similar conditions, for in none of the existing fishes are the fore and hind limbs alike. One of the most striking instances of a loss of parts, as well as of fusion of parts, occurs in the wing of the bird (q.v.), where are present a humerus, a radius and ulna, and two separate carpal bones only. The metacarpals are represented by two bones fused at their extremities and by a small bone on the radial side. Still distal to these are two rows of bones, one composed of two pieces and one of one piece only. The fossil bird Arclne opteryx had three or four fingers. The fourth and fifth phalanges have dropped out entirely or are inextricably fused with the other carti lages. In mammals two toes, the third and fourth. remain in artiodactyls (ox, etc.), and in perissodactyls (horses) only one, the third, per sists, but in fossil horses (q.v.) all intermediate stages from a five-toed condition have been dis covered. In man a number of cases of supernu merary parts (polytlactylism) occur. This is a highly inheritable character, regarded by Gegen baur as a monstrosity, but by Bardeleben consid ered as a case of atavism. Such a six-fingered condition is found in the adult in some amphib ians and reptiles, and also in a rodent (Pedetes). The human carpals are eight in number, arranged in two rows. The tarsal bones are seven in num ber.
The pectoral girdle arises ontogenetically later than the free appendages. In mammals, how ever, this part is characterized by a reduction in the ventral pair of pectoral girdle bones. which may result in their entire absence. The coracoid is lost wherever the movement of the arm is re stricted to an ambulatory one, since in carnivores and ungulates the clavicle is wanting. In such mammals as use their fore feet for digging, flying, or feeding the clavicle persists. In man the outer surface of the sternum is provided with a prominent ridge, the `spine of the scapula,' which runs out into a prominent process, the `acromium2 See PELVIS.
The human skeleton is composed of 200 distinct bones, exclusive of the 32 teeth and the ossicles in each tympanum. It is divided into four re gions: ( I) the skull, composed of 22 bones; (2) the trunk, composed of 54 bones; (3) the upper extremities, composed of 64 bones; and (4) the lower extremities, composed of 60 bones.