218

LIMBS OF THE TORTOISE AND TURTLE.

In the true or land tortoises the temporal depressions are exposed, as in the boxtortoises and fresh-water terrapencs: the head is proportionally small, and can be withdrawn beneath the protective roof of the carapace. The skull is rounder and less depressed than in the terrapenes: the frontals enter into the formation of the orbital border. The tympanic hoop is notched behind, but the columelliform stapes passes through a small foramen. The palatine processes of the maxillaries are on a plane much below that of the continuation of the basis cranii, formed by the vomer and palatines. In most of the chelonia the nasal bone is connate with the prefrontal; and, in all, the tympanic pedicle is firmly wedged between the broad appendage of the maxillary arch, formed by the malar, 26, and squamosal, 27, in front, and the mastoid, 8, behind. The broad-headed terrapene (podocnemys expansa) differs from other fresh-water tortoises, and approaches the marine tortoises (turtles), by the vaulted bony roof arching over the temporal depressions. This roof is chiefly formed by the parietals, but differs from that in the turtles in being completed laterally by a larger proportion of the squamosal than of the postfrontal, which does not exceed its relative size in other terrapenes. The present species further differs from the marine turtles in the nonossification of the vomer and the consequent absence of a septum in the posterior nostrils; in the greater breadth of the pterygoids, which send out a compressed rounded process into the temporal depressions: the orbits also are much smaller, and are bounded behind by orbital processes of the postfrontal and malar bones: the mastoids and paroccipitals are more produced backwards, and the entire skull is more depressed than in the turtles.

The ordinary position of the scapular extremity is a state of extreme pronation, as shown in Fig. 20, with the olecranon, or top of 54, thrown forwards and outwards, and the radial side of the hand, or thumb, i, directed to the ground. The humerus, 53, is strongly bent in a sigmoid form, with the anconal surface convex and directed upwards and outwards: the two tuberosities at the proximal end are much developed and bent towards the palmar aspect, bounding a deep and wide groove: that which answers to the external tuberosity is the smallest, and by the rotation of the humerus it becomes the most internal in position. The proximal row of the carpus consists of four bones-viz., a large scaphoides, a small lunare, wedged into the interspace of the radius and ulna, a large cuneiforme, and a small pisiforme. The second row consists of five distinct bones, corresponding with the five digits; those supporting the fourth and fifth answering to the os unciforme, the remaining three to the trapezium, trapezoides, and magnum. The first and fifth of the digits have cach one metacarpal and two phalanges; the rest, ii, iii, iv, have each a metacarpal and three phalanges. A sesamoid bone is placed beneath the metacarpo-phalangeal joint of the three middle digits. In the pelvic extremity, the femur, 65, is sigmoidally bent, but in a less degree than the humerus, and is a shorter bone. The patella is ligamentous: the synovial joint between it and the femur is distinct from the proper capsule of the knee-joint; the fibula, 66, is longer and more slender than the tibia, 66; a small "fabella" is articulated to its upper end. The proximal row of the tarsus consists of two bones, astragalus and calcaneum, which sometimes become confluent. The distal row consists of five bones, four of which support the four normal toes, and the fifth, a rudiment of the fifth toe without a claw: the fourth and fifth of the second row of tarsals answer to the os cuboides of higher animals; the other three bones to the three ossa cuneiformia. The astragalar part of the single proximal bone would seem to include the scaphoid as well as the calcaneum.

In the marine chelonia the digits of both limbs are elongated, flattened, and united by a web; the hands and feet having the form of fins.

In all the chelonia the long bones of the limbs are solid, without medullary cavities.

The Skeleton of Birds. From the massive frame of the cold-blooded, heavy, and proverbially slow tortoise, to the light, hot-blooded, flying bird, the transition seems to be abrupt, and the discrepancy between creatures so differently endowed extreme; nevertheless, at the confines of the feathered class, we find some aquatic species, such as the penguin, incapable of flight, having the wings modified to act as fins, and much resembling those of the turtle; with the bones solid, and the feathers r s mbling scales. All birds, like tortoises, lay eggs, are devoid of teeth, and have their jaws sheathed with horn, and forming a bill or beak. Most birds, however, enjoy the faculty of flight.

If the student of comparative osteology will procure the skull of a rook, a hawk, a swan, or a sea-gull, and vertically bisect it, he will have a ready instance illustrative of some of the characteristics of the osteology of the feathered class. Such a section will show the ivory-like whiteness and compactness of the osseous tissue, and the loose open cancellous structure of the bones. He will see that air is admitted into these cancelli partly from the nasal passages, and partly from the tympanic cavity which receives it from the eustachian tube; from the latter source, the proper bones of the cranium receive their air. Some of the characteristic features in the composition of the skull of birds may also be noticed: as, for example, the obliteration of all the ordinary sutures of the cranium, except those which unite the tympanic bone, 28, to the mastoid, 8; and that which unites the pterygoid, 23, to the basisphenoid, 5; which sutures are speedily obliterated in the human subject. The premaxillary is confluent with the nasal and with the maxillary; the nasal being confluent with the frontal and the maxillary with the jugal. The jugal and squamosal are also confiucnt, and form a long zygomatic style in all birds, connected at the hinder extremity by a moveable glenoid joint to the outer and lower part of the tympanic. The pterygoid articulates, in like manner, with the inner and lower part of the tympanic, the movements of which are thus communicated to the upper mandible, so far as the junction of the nasal with the frontal admits of such independent motion. The upper jaw, or mandible, which includes the vomer and nasals with the maxillary arch and appendages, is moveable in a bird through the junction of the nasals and nasal branch of the premaxillary with the frontal, by means of a moveable articulation, or by elastic plates.

If the student will next separate one of the vertebræ of the trunk from the rest, and cut out that portion of the long and broad breast-bone to which its pair of ribs are attached, he will have a segment of the skeleton, answering to that figured in Fig. 5, p. 169.

The cut surfaces will demonstrate the light cellulosity of the divided bones. The following letters indicate the elements of such modified vertebræ of the thorax: y, centrum, with its hypapophysis; p, parapophysis; d, diapophysis; n, neural arch and rudimental spine; pl, pleurapophysis; h, hæmapophysis; hs, hæmal spine. The tendency of individual elements and bones to coalesce in birds has already been illustrated in the cranium; it is shown, in most birds of flight, not only by the confluence of the centrum with the neural arch, but by that of several consecutive centrums and arches into a single bone, in the ample chest. In like manner the hæmal spines, which continue distinct in many vertebrata, have here coalesced into a single bone, which articu

lates on each side with the hæmapophyses of several vertebræ. These coalesced spines are also much developed in breadth, and send down, from the middle of their under surface, a longitudinal crest or keel. This modification relates to the extension of the surface for the origin of the great muscles of flight, and renders the "sternum," as the coalesced series of hæmal spines is called, one of the most characteristic parts of the skeleton of the bird. Ossification extends from the neural arches into the tendons of the vertebral muscles, and such bone-tendons, both here and in other parts of the body, as the legs, are also characteristic of birds. The scapula (Fig. 24), 51, is long and slender, as in the chelonia, but is more compressed and sabre-shaped. The coracoid, 52, as a general rule, is a distinct bone, moveably articulated to the scapula at one end

W

Fig. 24.-SKELETON OF THE SWAN (Cygnus ferus).

and to the sternum at the other. Its broad sternal end here articulates by a kind of gomphosis with a deep groove on the fore part of the sternum. The clavicle (ib.), 58.

articulates with the coracoid above, but is confluent with its fellow and with the keel of the sternum below. The iliac bones, 62, are remarkable for their length, and for the number of the vertebræ, or the great extent of the confluent spinal column, to which they are anchylosed. They reach in the swan, and in most other birds, from the tail forwards to the vertebræ with moveable ribs. Thus the artificial characters of a "lumbar vertebra" are wanting. The pubis and ischium on each side have coalesced with the ilium to form the lower boundary of the widely-perforated acetabulum. The pubis is long and slender, joins the ischium of its own side near its lower extremity, but does not join its fellow; thus the foramen ovale is defined, but there is no symphysis pubis : the absence of this symphysis facilitates the expulsion of the large ovum with its unyielding calcareous shell. The ischium coalesces posteriorly with the ilium, and converts the ischiadic notch into a foramen. The caudal vertebræ, Cd, are few in number, with broad transverse processes formed by confluent pleurapophyses, the limits of which may still be traced. A hæmapophysis is articulated to the lower interspace, between the fourth and fifth caudal, and is anchylosed to the sixth. The humerus of some of the larger birds of flight-e. g., the pelican or adjutant crane-is remarkable for its lightness, as compared with its bulk and seeming solidity; it is, in fact, a mere shell of compact osseous tissue. The orifice admitting air to its large cavity is beneath the great tuberosity at the proximal end.

The keel is excavated, not only for the reception of an air-cell, but likewise for a fold of the windpipe, which fold expands with age, and lies horizontally in the substance of the back part of the sternum. Small pneumatic foramina are situated at the anterior and inner surface of the bone, and perforate the articular surfaces for the sternal ribs.

In the skeleton of the wild swan (Cygnus ferus) (Fig. 24), here selected as an illustration of the ornithic modification of the vertebrate type, there are not fewer than twentyeight vertebræ, C S D, between the skull and the sacrum, the last six of which, DD, support moveable ribs: of these the first and second pairs are free; the next four are articulated to the sternum by bony hæmapophyses; the last five pairs of ribs are attached to the sacrum and also to the sternum; but the tenth, or last rib on the left side, is very rudimentary, being only about one inch in length. There are eight caudal vertebræ, Cd. The trachea or windpipe penetrates the sternum, and bends and winds in the interior of the bone before returning to enter the chest. The apex of the furculum, 58, bends upwards, and forms a hoop over the windpipe as it enters into the keel of the breast-bone. The furculum, sometimes called "merrythought," consists of the two clavicles confluent at their lower free ends. If a portion of the one side of the sternum be removed, the tortuous trachea which it incloses will be exposed. To the great length and peculiar course of the windpipe in this species is to be attributed its remarkably loud and harsh voice; whence the name hooper, or whistling swan, has been derived; and is applied in contradistinction to the domestic or mute swan, in which, as in most other birds, the trachea proceeds at once to the lungs, without entering the sternum. In the female of the wild species, the course of the trachea is much more limited than in the male, seldom penetrating the sternum to a greater extent than from three to four inches.

The breadth of the sternum, and the strong ridge or keel that descends from the mid-line of its under surface, relate to the increased extent of surface required for the attachment of the "pectoral" muscles, which are the active organs of flight. In the land-birds devoid of the power of flight, such as the ostrich and apteryx, the keel is

222

SKELETON OF THE DUCK TRIBE.

wanting and the sternum is short. Its various proportions, processes, notches, and per forations render it a very characteristic bone in birds.

In no order, founded upon modifications of the feet, is the sternum more diversified in character than in the palmipedes or web-footed order; for in none are the powers of flight enjoyed in such different degrees, or exercised in such various ways, from the frigate-bird down to the penguins, where the power of flight is abrogated, and the rudimental wings used as fins.

In the goose and duck tribes, as well as the swans (anseres, Linn.), the sternum is long and broad, and presents two moderately wide and deep hind notches; the costal processes are usually subquadrate; the coracoid grooves are continued into one another at the median line; the costal tract forms about half of the lateral margin in the ducks and geese, and two-thirds or more in the swans; the interpectoral ridge extends from the prominent part of the coracoid margin backwards, nearly parallel to the lateral margin, to the inner side of the lateral grooves; the back part of the sternum between the grooves is quadrate, with the angles slightly produced in most; there is a short manubrial process below the coracoid groove. The form of the sternum, its long keel, and the backward production of the long and slender ribs, give a boatlike figure to the trunk of these swimming-birds which is well adapted to their favourite medium and mode of locomotion. The bones of the wing or anterior extremity do not present that extraordinary development which might be expected from the powers of the member of which they form the basis. The great expanse of the wing is gained at the expense of the epidermoid system (quills and feathers, like hairs and scales, are thickened epiderm), and is not exclusively produced by folds of the skin requiring elongated bones to support them, as in the flying-fish, flying-lizards, and bats. The wing-bones of birds are, however, both in their forms and modes of articulation, highly characteristic of the powers and applications of the muscular apparatus requisite for the due actions of flight. The bones of the shoulder consist on each side of a scapula, 51, a coracoid, 52, and a clavicle, 58, the clavicles being, as a general rule in birds, confluent at their median ends, and so forming a single bone called "furculum" or 66 'os furcatorium;" this further modification of the hamal arch in birds, repeating that of the pubis and lower jaw in some other animals, having occasioned an additional specific term in ornithotomy. The scapula, 51, is a long, narrow, flat sabre-shaped plate, expanded at the humeral end, where it forms externally part of the joint for the arm-bone called "glenoid cavity," and extended backwards nearly parallel with the vertebræ, as far as the ilium, 62, in the swan, and reaching to the last rib in the swift; but it is much shorter in the birds incapable of flight. The coracoid is the strongest of the bones of the scapular arch: it forms the anterior half of the glenoid cavity, extends above this part to abut upon the furculum, and is continued downwards below the joint, expanding, to be fixed in the transverse groove at the fore part of the sternum; it thus forms the chief support of the wing, and the main point of resistance during its downward stroke. In the hawks and other birds of prey, and in the crows and most passerine birds, a small bone (os humero-capsulare) extends between the scapula and coracoid along the upper part of the glenoid cavity; this is absent in the swan and other swimmers, as well as in the gallinaceous and wading birds. The humerus, 53, is usually a long and slender bone, but is not always developed in length in proportion to the powers of flight; for, although it is shortest in the struthious birds and penguins, it is also very short, but much thicker and stronger in the swift and humming-birds. The head of the humerus is transversely oblong and convex; it is further enlarged by two