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SIXTH. See INTERVAL,

SIX'TUS. The name of five popes. SIXTUS or XYSTUS I., Saint, Pope c.116-25, under the reign of Hadrian.-SIXTUS or Xystus II., Saint, Pope 257-258. Under him the communion between Rome and the North African churches, broken off by the controversy over heretic baptism (q.v.) under his predecessor Stephen I., was restored. He died a martyr under Valerian, three days before his devoted deacon, Saint Lawrence (q.v.).-SIXTUS III., Saint, Pope 432440. To him is due the restoration of the Liberian basilica (Santa Maria Maggiore), in which his work is extant to-day, as also in the nave of another basilica built by him, the present Church of San Lorenzo. He is said to have sent Saint

Patrick to Ireland. - SIXTUS IV., Pope 1471-84, Francesco della Rovere. He was born near Savona in 1414, and became general of the Franciscan Order in 1464. Paul II. made him a cardinal three years later and was succeeded by him as Pope. His nepotism is the worst blot upon the memory of his pontificate, and led indirectly, through the ambition of his brother Girolamo, to unfortunate connection with the political affairs of Florence. The Pope's eighteen-year-old nephew, Cardinal Sansoni-Riario, having been arrested in connection with the assassination of Giuliano de' Medici in the conspiracy of the Pazzi, Sixtus demanded his release of Lorenzo de' Medici and satisfaction for the execution of the Archbishop of Pisa, who was suspected of complicity. Interdict, excommunication, and war followed; but after Lorenzo had won over the Pope's ally, the King of Naples, peace was made in 1480, and the Papal forces set free to act against the Turks, who had taken Otranto. Complications with Venice were terminated in

favor of the Republic by the Peace of Bagnolo. Sixtus, regarding it as a bitter humiliation and already ill, died five days later. Many public works were furthered by him, of which the most famous is the Sistine Chapel; the Ponte Sisto also commemorates his reign. Taxation, both civil and ecclesiastical, was so increased to carry out these projects and to provide for the Pope's family that it contributed not a little to disaffection against the Church. Consult, besides the general histories of the popes, Frantz, Sixtus IV. und die Republik Florenz (Regensburg, 1880). SIXTUS V., Pope 1585-90, Felice Peretti. He was born in 1521 in the March of Ancona, the son of a poor gardener. Like Sixtus IV., he entered the Franciscan Order and rose to high dignities, becoming Bishop of Santa Agata in 1566 and cardinal in 1570. He had lived a quiet and retired life before his election as Pope, and surprised the world by the masterful vigor of his reign. He began by repressing disorder and exterminating bands of outlaws in the Papal States; reformed the administration of the law and the disposal of patronage; and entered on comprehensive projects for the moral and material improvement of Rome. He laid down new regulations for the college of cardinals, restricting its number to seventy, and organized the modern system of congregations (q.v.), reorganizing that of the Inquisition which already existed; at the same time he strongly disapproved the excessive rigor of the Spanish Inquisition as a State tribunal under Philip II. He published a new edition of the Septuagint, and

an edition of the Vulgate (1590) as ordered by the Council of Trent, which contained so many errors that it had to be recalled and its place supplied by another under Clement VIII. The troubles of the League in France and the growth of Protestantism in England and Germany caused him great anxiety until his death on

August 27, 1590. Many of the popular stories concerning him are derived from the Life by Gregorio Leti (1669), a thoroughly untrustworthy work, answered by Tempesti, a Franciscan, in 1755. The best modern Life is by Baron von Hübner (Leipzig, 1871); consult also Capranica, Papa Sisto (Milan, 1884).

SIZE. See GLUE, and GELATIN.

SJÖBERG, shebar-y', ERIK (1794-1828). Α Swedish poet, born at Ludgo, and known in literature as Vitalis. He was educated at the University of Upsala, in which town he afterwards lived as a private tutor before finally removing to Stockholm. His poems-erotic, religious, humorous, melancholy, and satiric, by turns-appeared separately between 1819 and 1826, but were collected after his death by Geijer (1828). In 1873 there was a new edition by Forselius, entitled Samlade skrifter of Vitalis. A German translation was published at Leipzig in 1843.

SKAGEN, skägen, CAPE, or THE SKAW. The most northerly point of Jutland, Denmark (Map: Denmark, D 1). It is a narrow, sandy spit on which stands a stone lighthouse 148 feet high. Near the extreme point of the cape is the busy little port of Skagen.

SKAGERRAK, skäger-råk'. An arm of the North Sea lying between the south coast of Norway and the peninsula of Jutland, Denmark, and washing also the coast of Sweden (Map: Denmark, C1). It is the connecting link between the North Sea and the Cattegat, and is about 130 miles long by 80 miles wide. It is shallow near Jutland, where the coast is lined with dangerous sand banks, but deepens northward, being 600 feet deep in the middle and over 2000 feet deep near the Norwegian coast. The latter, as well as the Swedish coast, is indented with numerous bays affording good harbors. The Skagerrak is subject to violent northwest storms.

SKAG WAY. The subport of entry in the southern district of Alaska, 202 miles north of Sitka, at the mouth of the Skagway River, on Lynn Canal (Map: Alaska, H 4). It is the terminus of the White Pass and Yukon Railroad, and of the Seattle and Skagway steamship lines. Skagway lies amid attractive scenery. It has a public library, a United States Government building, and three hospitals. An army post also is here. There are breweries, bottling works, and a lumber mill; but the city is chiefly important as the distributing point of supplies for the interior and the Yukon mining district. The government is administered by a unicameral council, elected annually, which chooses one of its number as mayor. Skagway was settled in 1897, and received its present charter in 1900. Population, in 1900, 3117.

SKALD (Icel., poet), or SCALD. The name given in Old Norse specially to that class of poets who exercised their art as a vocation requiring a learned education; that is, a knowledge of the construction of verse, and of the enigmatical imagery, roughly shaped out of obscure tradition, to which Scandinavian poets were prone. The great, if not the only aim of the Skaldic poets was to celebrate the deeds of living warriors or of their ancestors. For this reason princes attached skalds to their courts, and competed with each other, by magnificent presents, for the possession of the most skillful minstrels. See ICELANDIC LITERATURE.

SKAT. A game of cards, the most intricate and perhaps the most scientific of them all. Its origin was in Germany, and dates from about the beginning of the nineteenth century. The derivation of the name is obscure.

Thirty-two cards are used, but, unlike whist cards, they are not double-ended. Not only the face cards, but the spot cards as well, usually show fully executed figures. Three or four persons take part in the game, although but three are active players, one, the player, playing against the other two. Each player holds ten cards, two being laid aside in the 'skat.' The use of these two cards determines the two differ ent styles of playing. With the skat, it is a simple game, or it may be Tourné (an order to turn up one of the cards in the skat), the suit of which becomes trumps. Or it may be without the skat, in which latter case the varieties of the game are designated as Solo, Nullo, and Grando. The four suits of the cards are: Eicheln (acorn), the equivalent of clubs; Grün (green), the equivalent of spades; Roth (red), the equivalent of hearts; Schellen (the bells), the equivalent of diamonds in other cards.

The four suits have a graded value, clubs counting the best, spades second, hearts third, and diamonds lowest. The trumping value of the jacks, which constitute the best trumps, follows the same crder; after which come the ace, ten, king, queen, etc., of the trumps turned. The nine, eight, and seven have no value of their own before the players bid for the privilege of playing the game, the one offering to play in the highest suit securing the privilege. This same player, however, is under the necessity of scoring at least sixty-one points. With sixty, he loses; with thirty points he is cut (Schneider); with no count at all he is black (Schwarz).

Points are as follows: aces, 11; tens, 10; kings, 4; queens, 3; jacks, 2. After the jacks, the ace is next in value, followed by the ten, king, queen, nine, eight, and seven. The four jacks only are trumps in Grando, while in Nullo there are no trumps at all. Each player must follow suit; but where that is not possible, a trump or any other card may be played. The dealer is determined by dealing one card to each player, until a jack is turned up, the player receiving it dealing the first round. The player to the right of the dealer 'cuts,' after which the cards are dealt to the left, five cards to each of the three active hands (the dealer, should there be four players, remaining inactive, then two cards in the skat, and another five cards to each player. Calling or bidding is according to the following rule: The second hand begins the bidding by offering a game to the first hand; or, if the second hand elects, the third hand makes the offer, and if he passes, the first hand has the play. Where two equally high games are bid, the first hand has preference to the others, and the

second to the third. No player may play a game of less value than his declared intention.

SKATE (from Icel. skata, skate; perhaps from Lat. squatus, squatina, sort of shark, angel-fish). The name of certain species of rays (q.v.). The commonest as well as the smallest species along the east coast of the United States is the tobaccobox (Raja erinacea); the largest is the barndoor skate (Raja lævis), four feet long. The big skate of California is the largest of the American skates, reaching a length of six feet, and its egg-case is nearly a foot long. The flesh, though coarse, is eaten, especially by Europeans. See Plate of RAYS AND SKATES.

SKATING (from skate, from Dutch schaats, ODutch, schaetse, high-heeled shoe). One of the primitive methods of man's progression over the ice when it is free from snow. The earliest form of skate was a shin or rib bone of some animal, tied to the skater's foot. Skates of bone are in the Guildhall collection in London and in other museums. The wooden skate shod with iron appeared in the fourteenth century. With the development of a metal foot piece bearing a cutting edge the art of progressing without the aid of the stick was acquired, the blade being set within a base of wood, which was strapped to the foot. Holland is still the paradise of skaters, and skating there, aside from its practical uses, is a national sport. Other notable skating countries of Europe are Russia, Norway, and Germany. Skating is very popular in Great Britain, and some famous skaters have been produced, especially from the Fens, on the eastern coast of England. The United States and Canada, with their long, cold winters, have produced many fast skaters who vie with the best of those abroad. Few outdoor sports in these countries attract so many devotees from the mass of the people. On the Hudson River have been made some of the fastest skating records, although Minnesota and the Middle West generally now rank with it. Montreal is the centre of Canadian skating. In 1884 a national amateur association was formed, with W. B. Curtis as president, and this has held successful championships ever since. Afterwards Eastern and Western sectional championships were instituted, and in 1899 the distances were measured according to the meter system, in accordance with the custom abroad. Foreign skaters in the United States have, as a rule, had to take second rank to the Americans.

The development of the skate used in the United States embraces three distinct periods. The old-fashioned skate had a straight, thick blade, sometimes with a double edge (gutter), affixed to a piece of wood, the skate being bound on by straps. Then came the club-skate, an improvement in that it was entirely of metal and could be instantly clamped to the foot. The blades were of a 'rocker' shape from end to end, which allowed fancy skating, but which reduced the speed in straightaway skating. Finally the Hudson River' or 'Donoghue' skate was introduced, which at once found favor in the West especially. This was practically a return to the old-fashioned form, the skate being straight-bladed and having a wooden top, with straps. The blade is long, projecting behind and before the foot, and very narrow, and the 'club' or foot-piece, when properly made of apple-wood, is very light. With it has come into popular favor the Norwegian skate, the best skate known, which has a similar blade, fastened permanently to the shoe by three metal pieces screwed to the sole. Its weight is but a few ounces. The hockey skate, a combination of the club and the Norwegian form, namely a short, thick, and straight-bladed skate screwed to the shoe, is another popular form.

The style of skating in America has been not a little influenced by the speed-skate, which by its nature has added considerable grace to the stroke. The principle of this stroke

is a gentle falling of the body from side

to side, as either skate takes its position for

the beginning of a stroke. The foot is s pushed almost straight ahead, the blade striking the ice flatly, instead of beginning, as in the club-skate, with the toe, and ending, at the finish of the stroke, with the heel. In pushing off, therefore, with either foot, the whole length of the blade is obtained as a purchase instead of the toe only, as in the case of the club-skate. The result is the greatest imaginable ease in skating,

while the length of the stroke is two to three times as long, saving considerable energy.

The competitions in figure-skating in the United States are under the control of the National Association, founded in 1885, which acts in conjunction with the Canadian Amateur Skating Association, founded in 1888, and the compe

titions for the championships are held annually, alternately in New York and Montreal.

SKAW, ska, THE. The most northerly point

of Denmark. See SKAGEN, CAPE.

SKEAT, WALTER WILLIAM (1835-). An English philologist. He was born in Park Lane, London, but passed his boyhood in Sydenham, a London suburb, then well in the country. It was here that he became familiar with the Kentish dialect. He attended King's College School, a school at Highgate, and entered Christ's College, Cambridge, where he graduated in 1858, and two years later was elected

a

fellow.

Ordained to the ministry in 1860, he held two curacies, first at East Dereham in Norfolk, and then at Godalming in Surrey; but, owing to an affection of the throat, he was compelled to give up the ministry. He returned to Cambridge, and resumed his studies in English philology and lit erature. In 1873 he helped to found the English Dialect Society, becoming its first director and afterwards its president. He had already begun editing Middle English texts for the Early English Text Society, established by his friend F. J. Furnivall. In 1878 he was appointed to the Erlington and Bosworth professorship of Anglo-Saxon at Cambridge, and in 1883 he was reëlected fellow of Christ's College. Among his separate publications may be mentioned The Songs and Ballads of Uhland, translated from the German (1864); Lancelot of the Laik (1865; revised 1870); the three texts of Langland's Piers the Plowman (186584; reprinted together 1886); An Etymological Dictionary of the English Language (187984); A Concise Etymological Dictionary of the English Language (1882); Barbour's Bruce (1870-77; and for the Scottish Text Society, 1893-94); Complete Works of Geoffrey Chaucer (1894); The Student's Chaucer (1895); A Student's Pastime, being a select series of articles

on

reprinted from Notes and Queries (1896); The Chaucer Canon (1900); Place Names of Cambridgeshire (1901); and Notes English Etymology (1901). Skeat is one of the leading scholars in the revival of our older literature, and has done much to popularize his subject. To him more than to all others is due the very general interest in Chaucer.

SKELETON (from σκελετόν, skeleton, mummy, dried body, neu. sg. of σκελετός, skeletos, dried, from σκέλλειν, skellein, to parch, dry up). The framework of hard structure which protects and supports the soft tissues of animals. The skele

ton either lies outside the soft tissues (exoskele

ton), or is imbedded within them (endoskeleton).

EXOSKELETON. Exoskeletal structures surround and shield the vital organs and muscles and are

represented by the shells or chitinous covering of mollusks, insects, and crustaceans, the shields of turtles, and the hair, scales, feathers, nails, and hoofs (qq.v.) of other vertebrates; also by the socalled 'membrane bones' of the skull. Phylogenetically the exoskeleton of vertebrates is older than the endoskeleton, and its structures were derived from the inner layer of the epidermis.

ENDOSKELETON. Endoskeletal structures appear in a few invertebrates (as the cuttlefishes, certain annelids, etc.), but are highly characteristic of vertebrates, in which arises a wholly new tissue arise from two sources, the endoderm and the

-bone. Endoskeletal structures of vertebrates

mesoderm, and are either membranous, cartilaginous, or bony. In the lower vertebrates the conversion of cartilage into bone takes place on the outside and proceeds inward. In the higher vertebrates ossification also takes place at certain internal centres. In the conversion of cartilage into bone the chondrin or matrix of the cartilage becomes converted into a calcified matrix. The matrix is then dissolved away by certain cells called osteoclasts. Around the walls of the cavities thus produced certain cells, osteoblasts, arrange themselves in a layer and secrete about themselves salts (carbonate and phosphate) of lime. The spaces occupied by these cells and their ameboid processes become much restricted, but persist as the 'lacune' and 'canaliculi' of bone. This calcified layer is in turn covered over by another internal layer of osteoblasts, and these in turn by others, until a Haversian system with its concentric layers is produced. Bone is always thus being torn down by the osteoclasts and made over by the osteoblasts. See BONE.

The skeleton of vertebrates may be treated under two heads: (1) the axial and (2) the appendicular skeleton.

VERTEBRAL COLUMN. The axial skeleton includes the vertebral column, ribs, sternum, and head-skeleton. The vertebral column, or backbone,' first appears in cyclostomes, where it occurs as fibrous tissue, surrounding the notochord, which thus comes to lie as a rod in the axis of the primitive vertebrate column and is known as the skeletogenous layer. From this point on it becomes a more and more important tant organ, while the chorda takes less and less part in the composition of the body of the adult. In the lowest vertebrates the skeletogenous layer is replaced at intervals by cartilage, which forms arches around the neural canal. In ganoids and higher forms these consist of five cartilaginous pieces for each somite, the fifth or unpaired piece forming the dorsal spinous process. Ventral cartilaginous pieces also occur ventral to the chorda. The bodies (centra) of the vertebræ definitely appear, and the chorda becomes constricted in travertebrally, giving the vertebræ an hour-glass form. The rings of cartilage formed by intravertebral constrictions are biconcave or 'amphicœlous' in all fishes with bony vertebræ and in most Urodela; also in a few fossil and living reptiles (q.v.), and in a few fossil birds. So long as the separate vertebræ of the vertebral column are amphicælous their connection with one another must depend upon something else than the bony vertebræ themselves. In the lower fishes this union is effected by the chorda and chordal sheath. In the lower Urodela it is effected by the intervertebral, non-ossified cartilage. In the higher Urodela, the Anura and almost all the reptiles, however, the vertebræ are linked to gether by means of a ball-and-socket joint. The concavity may be on the posterior and the convexity on the anterior end (opisthocœlous) or conversely (procœlous). In crocodiles, birds, and mammals the opposed faces of the vertebræ are approximately plane surfaces. In the development of the vertebræ of man the phylogenetic stages are recapitulated. The typical vertebra of man consists of a centrum from which an arch arises dorsally to protect the spinal cord. These arches together constitute the neural canal. Each half arch is composed of the rounded pedicle and the broad flat lamina. There are three kinds of processes: (1) the dorsal or neuropophysis; (2) the transverse process, serving for the attach ment of the muscles which keep the vertebræ together; (3) the forward and backward articulating processes (zygopophyses). The relation of the centra to the somites of the body is an interesting one. They do not arise one in the middle of each somite, but at the plane of separation of adjacent somites, thus insuring flexibility in

the column.

sess

The number of vertebræ in mammals is highly variable in different species. With one or two exceptions all mammals have seven cervical (nonrib-bearing) vertebræ. All the artiodactyls posnineteen thoracico-lumbar vertebræ. The smallest number (fourteen) occurs in armadillos; the largest (thirty) in hyracoids. Since the number of vertebræ corresponds to that of the somites of the body, it seems necessary to conclude that the latter are highly variable in number. If we seek for an interpretation of the differences in the vertebral column we may find it in the different tasks the parts perform, and the differentiation of vertebræ is a late acquisition, gradually acquired with advancing age. The sacral bones begin their fusion only at sixteen years, and this is not completed until the age of thirty. The sacrum is composed of four or five caudal vertebræ fused together.

RIBS. Ribs are also a part of the axial skeleton. Ventrally they end in cartilage and dorsally in two articular surfaces. The main part of the bone is the 'shaft' or 'body,' and its dorsal articular surface the 'head;' on the side near the head is a second articular surface, the 'tuberosity;' between this and the head there is a constriction, the 'neck.' In man the last of the normally twelve ribs is occasionally reduced to an insignificant rudiment, or a thirteenth rib may be

present. The transverse process of the seventh cervical vertebra and that of the first thoracic are quite different. The ventral arm of the transverse process of the seventh vertebra represents the rib. Similarly it may be inferred, even from the adult conditions, that all the cervical and trunk vertebræ possess ribs or the rudiments of ribs; and embryology bears out this conclusion.

STERNUM. The sternum or breast-bone' of man is a flat bone to which the ventral ends of the ribs are attached. Its anterior part is known as the 'manubrium,' the middle part as the 'gladiolus,' and the posterior cartilaginous tip as the 'xiphoid' or 'ensiform appendix.' The middle part is composed of more than one piece. In nearly all the lower mammals it is made up of as many bones as there are pairs of ribs attached to it, and this composition may be plainly seen in the sternum of a child. Moreover, the sternum of the young of many mammals shows a double origin, and it is plain that the sternum, if a product of the fusion of the ventral ends of the thoracic ribs, was originally laid down as a paired structure. The sternum of lower vertebrates is often closely united to the shoulder-girdle and possesses an accessory bone-the episternum. The sternum of Amphibia is small and the ribs do not meet ventrally. The sternum of most carinate birds is strongly keeled to permit of the attachment of powerful muscles of flight. See BIRD.

SKULL. We may distinguish in the skull the cranium or braincase and the visceral skeleton. In the development of the human skull three stages may be distinguished which correspond with phylogenetic stages: (1) The fibro-connective tissue stage. This is represented in phylogeny by the condition in Amphioxus, where a fibrous cordal sheath surrounds the notochord. (2) The cartilaginous stage. In the anterior region of adult selachians a large cartilaginous capsule, open above, completely surrounds the brain below and laterally, derived from two pairs of cartilage plates. Ver Ventral to the skull the visceral skeleton arises, consisting of the upper and lower jaws and the six branchial arches, the foremost of which early differentiated itself from the other five, entered into connection with the lower jaw, and constitutes the hyoid arch. The lower jaw arises in a manner precisely equivalent to a typical gill-arch, and is composed of two pieces on each side, the quadrate and Meckel's cartilage. Very 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

KEY TO SKELETON PLATE.

1. Frontal bone: 2. Parietal bone. 3. Temporal bone. 4. Occipital bone.

5. Malar bone.

6. Superior maxillary. 7. Inferior maxillary. 8. Cervical vertebræ. 9. Nasal bone.

10. Sternum.

11. Humerus. 12. Ulna.

13. Radius.

14. Lumbar vertebræ,
15. Innominate bones.

a. Phalanges.
b. Metacarpals.
c. Trapezium.
d. Scaphoid.

16. Sacrum.

17. Head of femur.

18. Shaft of femur.

19. Patella.

20. Shaft of tibia.

21. Fibula.

22. Greater trochanter of

femur.

23. Condyles of femur.

24. Tuberosity of tibia.

25. Clavicle.

26. Condyles of humerus.

27. Head of radius.

28. Dorsal vertebræ.

29. Scapula.

HAND.

e. Unciform.

f. Trapezoid.

g. Pisiform.

Foot.

m. Cuboid.

n. Navicular.

o. Ectocuneiform,

p. Mesocuneiform.

1. Entocuneiform.

h. Astragalus.
1. Calcaneum.
1. Metatarsus.
k. Phalanges.

and gills, forming the 'operculum.' Even in the Amphibia the bones of the skull preformed in cartilage can be artificially separated from dermal bones, but the higher we go in the vertebrate scale the more intimate is the union, 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 compact. Of the branchial apparatus there remains the first, the mandibular, 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 apparatus, see TЕЕТН.

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 currency. 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 shoulder and pelvic girdles have each been derived from one gill-arch and that the appendages are modified gill-rays-the bony processes of the gillarches, 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 condition, which occurs in Ceratodus, may be derived 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-arch corresponds 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 derived from entirely different sources. Dohrn believes the limbs have arisen from a continuous fin, which is paired anteriorly, but fuses posteriorly to form an unpaired ventral fin that extends up over the tail to the mid-dorsal line. By a failure of the development of a part of this continuous fin two paired 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 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 peripheral, the free appendage. It seems probable that the free appendage was developed first, and that the girdle arose from the necessity of a firmer 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 appendages of higher vertebrates is the same, and the remarkable 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 Archæopteryx had three or four fingers. The fourth and fifth phalanges have dropped out entirely or are inextricably fused with the other cartilages. In mammals two toes, the third and fourth, remain in artiodactyls (ox, etc.), and in perissodactyls (horses) only one, the third, persists, but in fossil horses (q.v.) all intermediate stages from a five-toed condition have been discovered. In man a number of cases of supernumerary parts (polydactylism) occur. This is a highly inheritable character, regarded by Gegenbaur as a monstrosity, but by Bardeleben considered as a case of atavism. Such a six-fingered condition is found in the adult in some amphibians 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 number.

The pectoral girdle arises ontogenetically later than the free appendages. In mammals, however, 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 restricted 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 tympanum. It is divided into four regions: (1) 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,

BIBLIOGRAPHY. Besides general works on vertebrates, anatomy, and paleontology, consult:

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