SKELETON OF THE KANGAROO.

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continent, as, from its extent, it has been termed, is subject to droughts of unusual duration, and the parched up grass, ignited by the electric bolt or other cause, often raises a conflagration of fearful extent, and leaves a correspondingly wide-spread blackened desert. To the antelope, and other ruminants of tropical or warmer latitudes, swiftness of limb has been given, which enables them to migrate to river valleys, where the vegetation is preserved from the influence of the dry Fig. 44. season. Australia, however, is peculiar for its scanty supply of perennial streams; the torrents of the brief periods of rain are reduced to detached pools in the dry season, and these are parched up in the long droughts, leaving hundreds of miles of the country devoid of surface water. If, then, the parent herbivore could traverse the required distance to quench its thirst, or satisfy its hunger, the tender young would be unable to follow the dam. A modification of the procreative process has accordingly been superinduced, which characterizes the Australian mammals; the young are prema

SKELETON OF THE KANGAROO (Macropus elegans).

turely brought forth of embryonic size and helplessness, and are transferred to a pouch of inverted skin, concealing the udder; and in this marsupium, as in a well-stored vehicle, they are easily transferred by the parent to any distance to which the climatal conditions may compel her to migrate. The economy of this portable nursery, the requisite manipulation of the suckling young therein suspended from the teat, demand a certain prehensile power of the fore-limbs, a freedom of the digits, with some opposable faculty in them, and the possession of so much sense of touch as would be impossible were the digit to be incased in a hoof; the horny matter is accordingly developed only on the upper surface of the finger-end, and is in the form of a claw. But the unguiculate pentadactyle extremity-though a higher grade of structure in the progress of limbs-is not suited for the exigencies of the herbivore, and would have appeared utterly incompatible with an existence dependent on grazing in wild pastures, had we argued from knowledge restricted to the forms and structures of the hoofed herbivores of the Europao-Asiatic, African, and American continents. How, then, it may be asked, is this difficulty overcome in the case of a grazing animal, necessarily a marsupial, and consequently an unguiculate one? The answer need only be a reference to fig. 44: the requisite faculty of migration of the parent with the tender offspring is gained by transferring the locomotive power to the hinder pair of limbs extraordinarily developed, and aided by a correspondingly powerful tail; the fore-limbs being restricted in their development to the size requisite for the marsupial offices and other accessory uses. This is the condition or explanation of the seemingly anomalous form and proportions of the kangaroo,-so strange, indeed, that the experienced naturalists, Banks and Solander, may well be excused for surmising they had seen a huge bird when they

254

CONDITIONS OF MARSUPIAL STRUCTURE.

first caught a glimpse of the kangaroo in the strange land which they, with Cook, discovered.

The rapid course of the kangaroo is by a succession of leaps, in which twenty to thirty yards are cleared at a bound; the herbivore, instead of a swift courser on four pretty equally developed hoofed extremities, is, in Australia, a leaping animal; and the saltatorial modification of the mammalian skeleton is here shown in that of one of the swiftest and most agile of the numerous species of kangaroo, the Macropus elegans. In this kangaroo, 13 vertebræ are dorsal, 6 are lumbar, 2 are sacral, and 28 are caudal, the first fourteen of which have hæmapophyses. These elements coalesce at their distal ends, and form small hæmal arches; they overspan and protect from pressure the great blood-vessels of the tail, the powerful muscular fasciculi of which derive increased surface of attachment from these hæmal arches. The pelvis is long; the strong prismatic ilia, 52, and the ischia, 63, carry out the great flexors and extensors of the thigh to a distance from their point of insertion-the femur, which makes these muscles operate upon that lever at a most advantageous angle; the trunk, borne along in the violent leaps, needs to be unusually firmly bound to the pelvic basis of the chief moving powers. Accordingly, we find a pair of bones, 64', extending forwards from the pubic symphysis, 64, along the ventral walls, giving increased bony origin to the unusually developed median abdominal muscles attaching the thorax to the pelvis; and these "marsupial bones," as they are called, have accessory functions relating to reproduction in both sexes of the marsupial quadrupeds. The femur, 65, is more than twice the length of the humerus: it is proportionally strong, with well-developed great and small "trochanters," and a "fabella" behind one or both condyles. The patella is unossified. The fibula, 67, is immoveably united to the lower half of the tibia. This bone, 66, is of unusual length and strength, and is firmly interlocked below with the trochlear astragalus. The heel-bone sends backwards a long lever-like process for the favourable insertion of the extensors of the foot. This member is of very unusual length. The innermost toe, or hallux, is absent; the second and third toes are extremely slender, inclosed as far as the ungual phalanx in a common fold of integument, and reduced to the function of cleansing the fur. The offices of support and progression are performed by the two outer toes, iv and v, and principally by the fourth, which is enormously developed, and terminated by a long, strong, three-sided, bayonet-shaped claw; these two toes are supported, as usual, by the os cuboides, which is correspondingly large, whilst the naviculare and the cuneiform bones are proportionally reduced in size. The bones of the fore-limb, though comparatively diminutive, present all the complexities of structure of the unguiculate limb. The clavicle, 58, connects the acromion with the sternum, and affords a fulcrum to the shoulder-joint. The humerus, articulating below with a radius and ulna which can rotate on each other, developes ridges above both inner and outer condyles for the extended origin of the muscles of pronation and supination. The brachial artery pierces the entocondyloid ridge. The carpal bones, answering to the scaphoid and lunar in the human wrist, are here confluent. The digits are five in number, enjoy free, independent movements, and are each terminated by a sharp-curved claw.

Skeleton of the Quadrumana.-The sloth is an exclusively arboreal animal; its diet is foliage; it has but to bring its mouth to the leafy food, and the lips and tongue serve to strip it from the branches. The extremities, as we have seen, serve mainly to climb and cling to branches, and occasionally to hook down a tempting twig within reach of the mouth. There is, however, another much more extensive and diversified order of arboreal

SKELETON OF THE APE TRIBE.

Fig. 46.

255

mammals destined to subsist on the fruits and other more highly developea products of the vegetable kingdom than mere leaves. In the monkeys, baboons, and apes the extremities are endowed with prehensile faculties of a more perfect and varied character than in the sloths; and this additional power is gained by a full development of the digits in normal number, with free and independent movements, which in one of them-the first or innermost-are such as that it can be opposed to the rest, so that objects of various size can be grasped. This modification converts a foot into a hand; and, as the mammals in question have the opposable "thumb" on both fore and hind limbs, they are called "quadrumana," or four-handed. The rest of the limb manifests a corresponding complexity or perfection of structure; the trunk is adjusted to accord with the actions of such instruments, and the brain is developed in pro

SKELETONS OF ORANG (Pithecus satyrus) AND MAN.

portion with the power of executing so great a variety of actions and movements as the four-handed structure gives capacity for.

256

COMPARISON OF THE BONY STRUCTURE OF THE APE AND MAN.

In the skull of the quadrumana are seen indications of a concomitant perfection of the outer senses: the orbits are entire, and directed forwards, with their outlets almost on the same plane; both eyes can thus be brought to bear upon the same object. The rest of the face, formed by the jaws, now begins to bear a smaller proportion to the progressively expanding cranium. The neck, of moderate length, has its seven vertebræ well developed, with the costal processes large in the fifth and sixth: the dorsal vertebræ, twelve, in the species figured (Pithecus satyrus), show by the convergence of their spines towards the vertical one on the ninth, that this is the centre of movement of the trunk. The lumbar vertebræ are four in number: in the inferior monkeys they are seven, and the anterior ones are firmly interlocked by well-developed anapophyses and metapophyses. The sacrum is still long and narrow. The tail, in some of the lower quadrumana, is of great length, including 30 vertebræ in the red monkey (Cercopithecus ruber), in which the anterior ones are complicated by having hæmal arches. The clavicles are entire in all quadrumana. The humerus has its tuberosities and condyloid crests well developed. The radius rotates freely on the ulna. The wrist has nine bones, owing to a division of the scaphoid, besides supplementary sesamoids adding to the force of some of the muscles of the hand; the thumb is proportionally shorter in the fore than in the hind foot. The patella is ossified, and in most baboons and monkeys there is a fabella behind each condyle of the femur. The fibula is entire, and articulated with the tibia at both ends. The tarsus has the same number and relative position of the bones as in man; but the heelbone is shorter, and the whole foot rather more obliquely articulated upon the leg, the power of grasping being more cared for than that of supporting the body; the innermost toe forms a large and powerful opposable thumb.

There is a well-marked gradation in the quadrumanous series from the ordinary quadrupedal to the more bipedal type. In the lemurs and South American monkeys the anterior thumb is shorter and much less opposable than the hinder one; in the spider-monkeys it is wanting, and a compensation seems to be given by the remarkable prehensile faculty of the curved and callous extremity of the long tail. This member in the African and Asiatic monkeys is not prehensile, but the thumb of the fore-hand is opposable. In the true apes the tail is wanting, i. e. it is reduced to the rudiment called "os coccygis;" but the fore-arms are unusually developed in certain species, hence called "long-armed apes." These can swing themselves rapidly from bough to bough, traversing wide spaces in the aërial leap. The orang (Fig. 45) is also remarkable for the disproportionate length of the arms, but this difference from man becomes less in the chimpanzees. The large species called Gorilla, which of all brutes makes the nearest approach to man, is still strictly "quadrumanous;" the great toe, or "hallux,' being a grasping and opposable digit. But the hiatus that divides this highest of the ape tribe from the lowest of the human species is more strikingly and decisively manifested in the skull (Fig. 50). The common teeth in the male gorilla are developed, as in the male orang, to proportions emulating the tusks of the tiger; they are, however, weapons of combat and defence in these great apes, which are strictly frugivorous. Nevertheless, the muscles that have to work jaws so armed require modifications of the cranium akin to those that characterize the lion, viz., great interparietal, 7, and occipital, 3, crista and massive zygomatic arches. The spines of the cervical vertebræ are greatly elongated in relation to the support of such a skull, the facial part of which extends so far in advance of the joint between the head and neck. The chimpanzees, moreover, differ from man in having thirteen pairs of thoracic moveable ribs.

ADAPTATION OF THE HUMAN SKELETON TO THE ERECT POSTURE. 257

The long and flat iliac bones, 62, the short femora, 65, so articulated with the legbones, 66, as to retain habitually a bent position of the knee, the short calcanea, c, and the inward inclination of the sole of the foot, all indicate, in the highest as in the lowest quadrumana, än inaptitude for the erect position, and a compensating gain of climbing power favourable for a life to be spent in trees.

In the osteological structure of man (Fig. 46), the vertebrate archetype is furthest departed from by reason of the extreme modifications required to adjust it to the peculiar posture, locomotion, and endless variety of actions characteristic of the human race. As there is nothing, short of flight, done by the moving powers of other animals that serpents cannot do by the vertebral column alone, so there is no analogous action or mode of motion that man cannot perform, and mostly better, by his wonderfully developed limbs. The reports of the achievements of our athletes, prize wrestlers, prize pedestrians, funambulists, and the records of the shark-pursuing and sharkslaying amphibious Polynesians, of the equestrian people of the Pampas, of the Alpine chasers of the 'chamois, and of the scansorial bark-strippers of Aquitaine, concur in testifying to the intensity of those varied powers, when educed by habit and by skilled practice. The perfection of almost all modifications of active and motive structures seems to be attained in the human frame, but it is a perfection due to especial adaptation of the vertebrate type, with a proportional departure from its fundamental pattern. Let us see how this is exemplified in the skeleton of man (Fig. 46), viewing it from the foundation upwards.

In the typical mammalian foot the digits decrease from the middle to the two extremes of the series of five toes; and in the modifications of this type, as we have traced them through the several gradations (p. 243, Figs. 35-39), the innermost, i, is the first to disappear. In man it is the seat of excessive development, and receives the name of "hallux," or "great toe;" it retains however, its characteristic inferior number of phalanges. The tendons of a powerful muscle, which in the orang and chimpanzee are inserted into the three middle toes, are blended in man into one, and this is inserted into the hallux, upon which the force of the muscle now called "flexor longus pollicis" is exclusively concentrated.

The arrangement of other muscles, in subordination to the peculiar development of this too, make it the chief fulcrum when the weight of the body is raised by the power acting upon the heel, the whole foot of man exemplifying the lever of the second kind. The strength and backward production of the heel-bone, c, relate to the augmentation of the power. The tarsal and metatarsal bones are coadjusted, so as to form arches both lengthwise and across, and receive the superincumbent weight from the tibia on the summit of a bony vault, which has the advantage of a certain elasticity combined with adequate strength. In proportion to the trunk, the pelvic limbs are longer than in any other animal; they even exceed those of the kangaroo, and are peculiar for the superior length of the femur, 65, and for the capacity of this bone to be brought, when the leg is extended, into the same line with the tibia, 66; the fibula, 67, is a distinct bone. The inner condyle of the femur is longer than the outer one, so that the shaft inclines a little outwards to its upper end, and joins a neck longer than in other animals, and set on at a very open angle. The weight of the body, received by the round heads of the thigh-bones, is thus transferred to a broader base, and its support in the upright posture facilitated. The pelvis is modified so as to receive and sustain better the abdominal viscera, and to give increased attachment to the muscles, especially the "glutei,” which, comparatively small in other mammals, are in man vastly developed to balance the trunk

ORGANIC NATURE.-No. IX.

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