lateral crests; of these the superior is the longest, and is bent outwards; the inferior is thickened and incurved, and beneath it is situated the orifice by which the air penetrates the cavity of the bone. The articular surface at the opposite or "distal" end is divided into two parts, one internal, for the ulna, of a hemispheric form, the other also convex, but more elongated and oblique, extending some way upon the anterior surface of the humerus. The extremity of a long bone of a limb which is next the trunk is called the "proximal" one; the extremity farthest from the trunk the "distal" one: they are not always "upper" and "lower." The ulna, 55, glides upon the inner hemispheric tubercle, upon the trochlear canal, and on the back part of the outer convexity. A ligament, extending from the outer part of the head of the radius to the outer part of the olecranon, above the posterior margin of the outer division of the articular surface of the ulna, plays upon the back part of the radial convexity of the humerus, and completes the cavity receiving it. The ulna is always stronger than the radius; but both are long, slender, and nearly straight bones, so articulated together as to admit of scarcely any rotation which adds to the resisting power of the wing in the action of flight. The upper part of the ulna, or "olecranon," is short. In the tendon attached to it a separate ossicle is developed in the swift, and two such. bones in the penguin. The ulna is often impressed by the insertions of the great quill-feathers of the wing.

The bones of the hand are very long and narrow, with the exception of the two distinct or unanchylosed carpal bones; these are so wedged in between the antibrachium, 54, 55, and the metacarpus, 57, as to limit the motions of the hand to abduction and adduction, or those necessary for folding up and spreading out the wing. The hand is thus fixed in a state of pronation; all power of flexion, extension, and rotation is removed from the wrist joint; so that the wing strikes firmly, and with the full force of the depressed muscles, upon the resisting air. The part of the hand numbered 57 in Fig. 24 includes the metacarpal bones of the digits answering to the second, third, and fourth of the pentadactyle members, which are confluent at their proximal ends with each other, and with the "os magnum," one of the carpal bones, now forming the convex base of the middle metacarpal. This metacarpal and that answering to the "fourth" digit are of equal length, and are also confluent at their distal ends; but the middle or "third" metacarpal is much the strongest. That answering to the "second" digit, ii, is very short, and like a mere process from the third; it supports two short phalanges in the swan. The third metacarpal supports three phalanges, iii, the fourth a single phalanx, iv. All these are wrapped up in a sheath of integument, and are strongly bound together; so that the wing loses nothing of its power, whilst so much of the typical structure of the member is retained, that every bone can be referred to its corresponding bone in the most completely developed hand.

In ornithology the large quill-feathers that are attached to the ulnar side of the hand are termed "primaria," or primary feathers; those that are attached to the forearm are the "secundaria," or secondaries, and "tectrices," or wing-coverts; those which lie over the humerus are called "scapulariæ," or scapularies; and those which are attached to the short outer digit, i, erroneously called the "thumb," are the "spuriæ," or bastard feathers. The bones of the leg do not present the same number of segments as those of the wing, that corresponding with the carpus being wholly blended with the one that succeeds.

The pelvic bones offer this contrast with those of the shoulder, that they are always anchylosed on either side into one piece, " os innominatum" and not at the median

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PELVIS AND BONES OF THE LEG OF BIRDS.

line, whilst this is the only place where the elements of the scapular apparatus are united by bone. In the young bird the os innominatum is composed of three bones. The ilium, 62, is flattened, elongated, usually anchylosed to a very long sacrum: it forms the upper half of the joint for the thigh-bone, called "cotyloid cavity." The pubis, 64, is very long and slender: it does not meet its fellow at the middle line in any bird save the ostrich, but is directed backwards, with its free extremity bent downwards. The pelvis of the ostrich is so vast, that the pubic junction completing it does not impede the exit of the egg; in other birds the open pelvis facilitates the passage of that large and brittle generative product. The ischium, 63, is a simple elongated bone, extending from the cotyloid cavity backwards, parallel with the ilium; it sometimes coalesces, as in the swan, with both the ilium and pubis at its distal end.

The cotyloid cavity is incomplete behind, and is closed there by ligament. The femur, 65, is a short, cylindrical, almost straight bone; the head is a small hemisphere, presenting at its upper part a depression for the "round ligament." The single large | "trochanter" generally rises above the articular eminence, and is continuous with the outer side of the shaft. The orifice for the admission of air is situated in the depression between the trochanter and head. The distal end presents two condyles, the inner one for the inner condyloid cavity of the tibia; the outer one for the outer cavity of the tibia and for the fibula; the outer condyle is produced into a semicircular ridge, which passes between the tibia and fibula: this ridge puts the outer elastic ligament on the stretch, when the fibula is passing over the condyle, and the fibula is pulled into a groove at the back of the condyle, with a jerk, when in extreme flexion; this springjoint is well exemplified in both the swan and water-hen.

The proximal end of the tibia is divided into the two shallow condyloid cavities above noticed: two ridges are extended from its upper and anterior surface: the strongest of these is the "procnemial" ridge, and is slightly bent outwards: the shorter one on the outside of this is the "ectocnemial" ridge; they are usually united above by a transverse ridge, called "epicnemial" ridge; this is developed into a long process in the divers, grebes, and guillemots: a fibular ridge projects slightly from the upper third of the tibia for junction with the fibula. The distal end of the tibia forms a transverse pulley or trochlea, with the anterior borders produced. Above the fore part of the trochlea is a deep depression, and in many birds an osseous bridge extends across it.

The third segment of the leg, 69, is a compound bone, consisting originally of one proximal piece, short and broad, presenting two articular concavities to the two thick and round borders of the tibial trochlea, of three metatarsals which coalesce with each other and with the above tarsal piece, and of one or more bony processes which are ossified from the back part of the proximal piece, or from the proximal ends of the metatarsals, and which, from their relations to the extensor tendons, are called "calcaneal" processes. In most birds a small rudimental metatarsal, supporting the innermost toe or "hallux," i, is articulated by ligament with the innermost of the coalesced metatarsals, and is properly included in the same segment of the limb. The three principal metatarsals are interlocked together before they become anchylosed, the middle one being wedged into the back part of the interspace of the two lateral ones above, and into the fore part below, passing obliquely between them. The period at which these several constituents of the "tarso-metatarse" coalesce is shorter in the birds that can fly than in those that cannot; and the extent of the coalescence is least in the penguins, in which the true nature of the compound bone is best seen.

STRUCTURE OF THE FOOT IN BIRDS.

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The modifications of the tarso-metatarse are chiefly manifested in its relative length and thickness, in the relative length of the three metatarsals, and in the number and complexity of the calcaneal processes.

The inner of the two cavities for the condyles at the proximal end of the bone is the entocondyloid" cavity or surface, the outer one the "ectocondyloid" surface; they are separated by an "intercondyloid" tract, from the fore part of which there usually rises an intercondyloid tuberosity. The entocondyloid cavity is usually the largest and deepest: it is so in the raven, in which the base of the intercondyloid tubercle extends over the whole of the intercondyloid space. There are three calcaneal processes: one, called the "entocalcaneal," projects from below the entocondyloid cavity, and from the back part of the upper end of the entometatarse; a second, called the "mesocalcaneal," from the intercondyloid tract and the mesometatarse, and the third called "ectocalcaneal," from behind the ectocondyloid cavity and the ectometatarse. These three processes are united together by two transverse plates circumscribing four canals, two smaller canals being further carried between the ento- and meso-calcaneal processes. The primitive interosseous spaces are indicated by two small foramina at the upper and back part of the shaft, which converge as they pass forward, and terminate by a single foramen at the fourth part of the anterior concavity. A similar minute canal is retained between the outer and middle metatarsals, near their distal ends; each metatarsal then becomes distinct, and developes a convex condyle for the proximal phalanx. The middle one is the largest, and extends a little lower than the other two; it is also impressed by a median groove; the more compressed lateral condyles are simply convex, and are of equal length. A rough surface, a little way above the inner condyle, indicates the place of attachment of the small metatarsal of the hallux.

In the swan and other anserine birds the calcaneal prominence presents four longitudinal ridges, divided by three open grooves, the innermost ridge being the largest; the shaft is subquadrate, with the angles rounded, and none of the surfaces are channelled. The inner condyle scarcely extends before the base of the middle one; the canal perforating the outer intercondyloid space is bounded below by two small bars passing from the middle to the outer condyle, and which bars define the groove for the adductor muscle of the outer toe.

The tarso-metatarse of the diver (colymbus) is remarkably modified by its extreme lateral compression. The ento- and ecto-calcanea are prominent, oblong, subquadrate plates, inclining towards each other, but not quite circumscribing a wide intermediate space. The broad outer and inner surfaces of the shaft are nearly flat; the narrow fore and back surfaces are channelled; the anterior groove leads to the wide canal, perforating obliquely the shaft above the outer intercondyloid space, from which a narrower canal conducts to that interspace. The middle and outer trochleæ are nearly equally developed; the inner one stops short at the base of the middle one.

The number of toes varies in different birds; if the spur of the cock be regarded as a rudimental toe (which is not, however, my view of it), it may be held to have five toes, while in the ostrich the toes are reduced to two. Birds, moreover, are the only class of animals in which the toes, whatever be their number or relative size, always differ from one another in the number of their joints or phalanges, yet at the same time present a constancy in that variation.

The innermost or back toe, i (Fig. 24), answering, as I believe, to the "hallux," or innermost digit of the pentadactyle foot, has two phalanges; the second toe, ii, has three, the third toe, iii, four, and the fourth toe, i, five phalanges; I believe the toc answer

ORGANIC NATURE.-No. VIII.

P

226

MECHANISM OF FLIGHT IN BIRDS.

ing to the fifth in lizards and other pentadactyle animals to be wanting in the bird s foot, and the spur, sometimes single, sometimes double, as in Pavo bicalcaratus, to be a superadded weapon to the metatarse. As the toes in the tridactyle emeu, cassowary, and bustard, have respectively three phalanges, four phalanges, and five phalanges, we recognise them as answering to the second, third, and fourth in other birds; the toss in the didactyle ostrich have respectively four and five phalanges, and what is here truly suggestive, the outermost, which is much the smallest and shortest toe, has the greater number of joints, viz., five, thus retaining its ornithic type, as the fourth, or outermost, toe.

The entire form of the body, and consequently that of its bony framework, in a bird, has special reference to the power of flight. The trunk is an oval with the large end forwards. The vertebral column of this part is short and almost inflexible, so that the muscles act to great advantage; the spine of the neck being long and flexible, the centre of gravity is readily changed from above the feet—as when standing or walking -to between and beneath the wings during flight; when suspended in the air the bird's body naturally falls into that position, which throws the centre of gravity beneath the wings. The axis of motion being situated in a different place in the line of the body when walking from that which is used when flying, the discrepancy requires to be compensated by some means in all birds, in order to enable them to perform flight with ease. Raptorial birds take a horizontal position when suspended in the air, and the compensating power consists in their taking a more or less erect position when at rest. Another class, including the woodpeckers, wagtails, &c., take an oblique position in the air; with these the compensating power consists in their cleaving and passing through the air at an angle coincident with the position of the body, and performing flight by a series of curves or saltations. Natatorial birds sometimes need very extended flight; they take a very oblique position in the air, stretch out their legs behind and their neck in front; they have the ribs greatly lengthened, the integuments of the abdomen are long and flexible, which enables them greatly to enlarge the abdominal portion of their bodies by inflating it with air; this causes a decrease in the specific gravity of that part, and raises it to a horizontal position; the compensating power consists in the posterior half of the body becoming specifically lighter, while the specific gravity of the anterior half remains unaltered. When they alight they drop the legs, throw back the trunk by bending the knee-joint, and bring the head over the trunk by a graceful sigmoid curve of the long neck, as in Fig. 24. The act of swimming is rendered easy by the specific gravity of the body, by the boat-like shape of the trunk, and by the conversion of the hinder extremities into oars, in consequence of the membranes uniting the toes together. The effect of these web-feet in water is further assisted by the toes having their membranes lying close together when carried forwards; whilst, on the contrary, they are expanded in striking backwards. The oar-like action of the legs is still further favoured by their backward position,-an arrangement, however, which is unfavourable for walking.

Borelli was the first who, by comparison of the anatomical peculiarities of the human frame and the structure of birds, demonstrated, to a certain extent, the impossibility of the realization of the cherished project of flying by man. He arrived at this conclusion from a comparison of the form and strength of the muscles of the wings of birds with the corresponding muscles of the human body.

Principal Forms of the Skeleton in the Class Mammalia.-In the class Mammalia, which includes the hairy quadrupeds with the naked apodal whales and

VARIOUS FORMS OF LIMBS IN MAMMALS.

227

biped man, the form of the animal is modified for a great diversity of kinds and spheres of locomotion. Some live exclusively in the ocean, and cleave the liquid element under the form and with the locomotive powers of fishes; some frequent the fresh waters; some pass a subterraneous existence, and work their way through the solid earth; some mount aloft to seek and seize their prey in the air; some pass their lives in trees; most, however, dwell on the earth, with various powers of walking, running, and leaping. Lastly, man is modified to sustain his frame erect on the hinder, now become in him the lower, limbs.

In the Mammalian class, accordingly, we find the limbs progressively endowed with more varied and complicated powers. They retain in the Cetacea (whale and porpoise tribe) their primitive form of flattened fins; in the Ungulata (hoofed beasts) one or more of the digits acquire the full complement of joints, but have the extremity enveloped in a dense hoof; in the Unguiculata (quadrupeds with claws) the limbs, with ampler proportions, have the digits liberated, and armed with claws confined to the upper surface, leaving the under surface of the toes free for the exercise of touch; in the mole the hand is shortened, thickened, expanded, and converted into a sort of spade; in the bat the fingers are lengthened, attenuated, and made outstretchers and supporters of a pair of wings; in the Quadrumana (ape and monkey tribes) certain digits are endowed with special offices, and by a particular position enabled to oppose the others, so as to seize, retain, and grasp. Lastly, in Man the offices of support and locomotion are assigned to a single pair of members; the anterior, and now the upper, limbs being left free to execute the various purposes of the will, and terminated by a hand, which, in the matchless harmony and adjustment of its organization, is made the suitable instrument of a rational being.

In contemplating and comparing the skeletons of a series of mammals, the most striking modifications are observable in the structure and proportions of the limbs.

There are a few osteological characters in which all mammalia agree, and by which they differ from the lower vertebrata; and some have been supposed to be peculiar to them that are not so. The pair of occipital condyles, e. g., developed from the exoccipitals, are a repetition of what we saw in the batrachia. The flat surfaces of the bodies of the trunk-vertebræ were a character of many extinct reptiles; but these surfaces in mammals are developed on separate epiphysial plates, which coalesce in the course of growth with the rest of the centrum. Moveable ribs, projecting freely (pleurapophyses) in the cervical region, may be found in a few exceptional cases (sloths, monotremes); bony sternal ribs (hæmapophyscs) exist in most Edentata; a coracoid extending, as in birds and lizards, from the scapula to the sternum, with an "epicoracoid,” as in lizards, is present in the monotremes (platypus or duck-mole, and echidna or spiny ant-eater, of Australia); the cotyloid cavity may be perforated in the same low mammals as in birds; the digits may have the phalanges in varying number in the same hand, and exceeding three in the same finger, e. g., in the whale tribe. But the following osteological characters are both common and peculiar to the mammalia. The squamosal, 27, or second bone of the bar continued backwards from the maxillary arch, is not only expended as in the chelonia, but develops the articular surface for the mandible, and this surface is either concave at some part or is flat; each half or ramus of the mandible is ossified from a single centre, and consists of one piece; and the condyle is either convex or is flat, never concave. The presphenoid (centrum of the parietal vertebra) is developed distinctly from the basisphenoid; it may become confluent, but is not connate, therewith.