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is developed in proportion to the muscular activity of the shoulder joint, and is consequently large in man.

The HEAD, like the rest of the body, is formed of several vertebrate segments, but greatly modified to suit the requirements of the brain and face. It is divided into two parts, the cranium and face. The cranium, placed behind and above, is the neural canal expanded to contain the brain, and rising forwards over the hæmal part. It is formed of eight bones-the frontal, at the forehead; the occipital, behind; the parietal, at each side and above; the temporal, on each side, above and around the ear; and, in the base, the sphenoid and ethmoid.

The Face is the hæmal, or food and air, part of the head; presenting a double inlet, the nose for air, the mouth for food and air. It is composed of fourteen bones. They are-1. The malar or cheek bones. 2. Nasal. 3. Lachrymal. 4. Inferior spongy. 5. Palate. 6. Superior maxillary, or upper jaw. All in pairs. To these twelve remain to be added-7. The vomer or ploughshare bone, in the partition between the right and left nasal cavities; and 8. The inferior maxillary, or lower jaw bone, which is loosely jointed, in order that it may move in mastication. Most of the facial bones are small and arranged around the nasal cavities; and much the larger part of the face is formed by the maxillary bones, which surround the mouth and support the teeth.

The distinction of cranium from face, is very important in recognising the characters of heads in different individuals and varieties of mankind. The form and size of the face-that is, from the eyebrows downwards—is influenced by the development of the nasal cavities, and of the jaws; while the cranium corresponds to the brain. (See Fig. 1.) In the child, the face is very small in proportion to the cranium, for the double reason that the brain is very early developed, while the jaws and nose are as yet small. The female face retains more of the child form than the male. In old age generally, the alteration of the face is mainly owing to the loss of the teeth, and the subsequent absorption of their sockets, shortening the face, sinking the mouth, and protruding the chin.

The LIMBS in all vertebrate animals are formed on the same type or plan, however much each may be modified to render it an appropriate instrument for motion in the air, or in the water, or for climbing, running, or burrowing. The upper and lower limbs,

therefore, correspond to each other, not only in divisions but in individual bones. These are modified so as to adapt the lower limb of man for supporting the body and for locomotion, and the upper for grasping; the main purpose of the erect posture being to set free the upper limb for use as a hand.

In the following table, the names are arranged so as to show the homology,1 or correspondence, between the bones of the two limbs (Fig. 3.)

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Thus each digit has three phalanges except the thumb and big toe, which have only two. In comparing the limbs, the hand must be brought into the prone position, that is, with the palm down; the thumb then corresponds to the big toe, and is the first or internal digit.

It will now be evident, from this table, and the accompanying diagram of a pattern and complete limb (Figs. 4 and 5), that the plan or idea of a limb is as simple as it is beautiful. That it is formed, as it were, by two kinds of subdivision: first, transverse division into a series of segments, or divisions, succeeding each

1 Homology (homos, like; logos, a description), tracing resemblance, or common type, amid varieties. Teleology (telos, use or purpose, and logos), tracing the adaptation of the plan to the use in the animal.

Carpus, the wrist. Tarsus, the ankle. Metatarsus (meta, beyond); beyond the ankle. Phalanx, a battalion or row. Digitus, a finger. Scaphoid, scaphè, a boat; eidos, like. Semi-lunar, halfmoon like. Cuneiform, cuneus, a wedge; forma, shape Pisiform, pea-like. Trapezium and Trapezoid, square-shaped, or like. Os Magnum, the largest bone. Unciform, hook-like Astragalus, ankle-bone. Os Calcis, heel-bone. Cuboid, like a cube or solid square.

other end to end, in all eight in number,-thus enabling the parts to be bent up or straightened on each other; and, secondly, of longitudinal subdivision, or splitting of a segment into two or more pieces, so as to allow the one to move upon the other, as in the fore-arm and leg, or of the separate motion of each, as in the case of the fingers and toes.

1

FIG. 4.

FIG. 5.

The first segment is always single, its office being to carry the entire limb about in all directions; and, to allow of this, the humerus and femur are always set on the body by ball and socket joints. The next joint, the elbow or knee, is a simple hinge, enabling the limb to be doubled up. The second segment presents two parallel bones, generally moving more or less upon each other. This allows the hand to be rolled up and down; and obviates shock in the transmission of force from the hand or foot. The patella, or knee-pan, belongs to a different class of bones, termed sesamoid, which are developed in the tendons of muscles. Two such bones exist, and may be felt, at the metacarpo-phalangeal joint of the thumb, and the corresponding joint of the great toe; and in many of the lower animals, at the other toes as well. The carpus or wrist, and tarsus or ankle bones, are groups of short irregular-looking bones, allowing of free motion on each other, and thus obviating shock or abruptness, under force; but however modified, they are always arranged in two rows, and form the third and fourth segments of the limb. The full typical number of five in each row is presented in some animals. A glance at the feet of the bear, or any other five-toed quadruped, will show that the os calcis of the ankle corresponds to the cuneiform and pisiform bones of the wrist, joined together; thus accounting for the apparent discrepancy. The remarkable fact of the absence of a third phalanx in the thumb and great toe, has no essential refer

Fig. 4. Pattern or type of a complete limb.

Fig. 5. The same wrought up into the human superior extremity.

1 Sesame, an Indian bean; eides, the shape.

ence to length, for, though it accords with the required shortness of the thumb, the great toe is not shorter than the other toes, and in most birds the longest toe has not the most phalanges. Light is thrown upon this, by the fact that in all five-toed quadrupeds the inner toe is the smallest and shortest, naturally accounting for the bone less; while, farther, this toe is the earliest to disappear, and first on the hind foot, in those animals which have fewer than five toes. It is, likewise, interesting to notice that this digit, which in quadrupeds is either small or wanting, is in man developed into the most useful and important of the five, the thumb and big toe; the former as an opposable finger for grasping; the other, still more characteristic of man than the thumb, as the great front support of a foot made for the erect posture. Yet Nature's law of the bone less is not departed from in man. Three phalanges, however, is not the extreme limit in vertebrate limbs. Additional segments are in some developed after the three ; as, among mammalians,1 in at least two of the fingers of the whale ; and, in birds, the usual four toes have successively from within outwards two, three, four, and five phalanges; and the many jointed rays of fishes may be regarded as an illustration of the same principle carried to a greater extent.

The material of which bones are made is a compound of soft animal matter and of various hard earthy or mineral matters, chiefly phosphate and carbonate of lime. The animal part may be driven off by burning, or decay, leaving the bone brittle and crumbling; and the earthy parts may be dissolved by acids, leaving the bone soft and flexible, like a rope. The relative proportions of the hard and soft constituents vary at different periods of life, the hard increasing with age. Hence the bones of the child are soft and may even bend without breaking, while in old age they become brittle and more liable to break.

The osseous tissue, thus composed, is arranged in the bones in two forms, the dense or solid looking, and the cancellated or spongy. In the latter, the bony tissue is opened out, so as to give bulk, without at the same time increasing the weight, as at the enlarged ends of the limb bones, and in all masses of bone.

In the shafts of the long bones of the limbs, the dense texture forms a hollow cylinder, thus combining lightness with the re

1 Mammalia are animals which suckle their young, the highest class of the highest or vertebrate division of the animal kingdom.

quired size and strength. This is the true reason why these bones are hollowed out, and not in order that they may contain marrow, which is merely fat, and is placed there because it is the lightest animal solid. The same principle is carried still farther in some of the bones of most birds, the interior of the large and thin-walled tube being filled with air, giving them greater bulk and strength for the same weight, and rendering them relatively lighter in the air. A similar arrangement exists in several parts of the human head, as in the sinus, or cavity, of the upper jaw, between the eye and mouth; in the mastoid cells behind the ear; in the ethmoidal and sphenoidal cells, between the eyes; and in the frontal sinuses at the lower third or fourth of the forehead, and backwards above the eyes. In these situations, in the adult, the bones are hollow and filled with air from the nose; thus giving the required external size, without rendering the head too heavy.

THE JOINTS.

THE parts concerned in the formation of a complete joint, or articulation, are—

1. The articular1 surfaces of the bones, of various forms in accordance with the kind of motion.

2. These encrusted by a layer of cartilage,2 elastic and naturally insensible, like a layer of India-rubber, serving to diminish the shock and friction which would have resulted from the forcing and rubbing of hard bones against each other.

3. A synovial membrane, lining the interior, like a closed oil bag, not only containing but secreting its own oil or synovia ; which lubricates the surfaces, like oil on machinery, rendering motion easy, and, in conjunction with the cartilage, preventing friction.

4. Ligaments by which, as by cords or tapes, the bones are tied together, serving more precisely to check and prevent the movements from going too far.

The ordinary ligaments are made of white fibrous tissue, as a cord is of threads, only not twisted; and are soft and flexible like cords, but not elastic. Had they been elastic, they would have yielded and allowed the joints to dislocate easily, an accident which

Artus, or Articulus, a joint.
Sun, together; ovum, an egg.

Like white of an egg.

2 Cartilago, gristle.
Ligo, to bind.

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