Page:The New International Encyclopædia 1st ed. v. 18.djvu/256

* SKELETON. 212 SKELETON. to a typical gill arch, and is composed of two pieces on each side, the quadrate and Meckel's cartilage. Verj- early a forward outgrowth from the quadrate gives rise to the upper jaw. (3) The bony stage is represented in the bony ganoids, where the frame-case is covered by enamel plates. Dermal bones also cover over the branchial arches and gills, forming the 'operculum.' Even in the Amphibia the bones of the sUull preformed in cartilage can be artificially separated from der- mal bones, but the higher we go in the vertebrate scale the more intimate is the tmion. until in mammals the two bones are developed at the same time and are inseparably fused in the adult. With the loss of gills goes that of the opercular apparatus, and the cranium becomes more com- pact. Of the branchial apparatus there remains the first, the niandibiilar, the second, the hyoid, and a part of the third, which fuses with the hyoid. Finally the axis of the cranium curves. The curve is first considerable in reptiles and birds and reaches its maximum in man. See Skull; for the anatomy of the bones of the ear. see Ear; and for that of the dental appai'a- tus. .see Teeth. The AppENDiciLAR Skeleton. Appendages in vertebrates may be divided into two kinds : ( 1 ) paired, and ('2) unpaired or median. Paired appendages are represented bj" 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 w'hich 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. Gegenbaur'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-merabrane. 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 atVorded 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-arch corre- sponds to one metamere, the appendages cannot be derived from gill-arches and their rays. More- over, the muscles of the appendages are derived from the dorsal muscle-plates and those of the branchial arches from the lateral plates of the head, hence the musculature of the two are de- rived from entirely different sources. Dohrn be- lieves the limbs have arisen from 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 line. 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 otV from the muscle-plate in the interappendieular region just as at the appendages; these muscles later degenerate. Dohrn also finds muscle-mas.ses given ofl' in each somite to the median fin. Hence the median fin is to be regarded as derived from two fused lateral fins. The paired appendages of vertebrates fall into two types: (1) that of fishes, and (2) 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 a.xial 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 pre.sent 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 Archae- opteryx had three or four fingers. The fourth and fifth phalanges have dropped otit 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 (polydactylism) 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 ninn- ber. The pectoral girdle arises ontogcnetically later than the free appendages. In mammals, how- ever, this part is characterized by a reduction in the ventral pair of jiectoral 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 'acromium.' See Pelvis. The human skeleton is composed of 200 distinct bones, exclusive of the 32 teeth and the ossicles in each tymiianiun. 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. BiBLioGRAPHV. Besides general works on ver- tebrates, anatomy, and paleontology, consult: