Part 37

g. g. Pelvic or Hip bones.

h. h. Humeri or Arm bones.

i. Radius } } Bones of fore-arm. j. Ulna }

k. Femur or Thigh bone.

l. Tibia or Large bone of leg.

m. Fibula or Small bone of leg.

n. Calcaneum or Heel bone.

o. Tarsal bones or Bones of the foot.

p. Carpal bones or Bones of the wrists.

FIG. 73.--THE HUMAN SKELETON.]

The Human frame consists of a skeleton (fig. 73) formed of hard and unyielding bone, having joints to admit of motion in certain directions; this skeleton performs certain great and important offices, it forms strong boundaries or protecting cases for most of the vital parts, so as to shield them from external violence, such as the brain and spinal cord, the lungs, heart, &c., it also furnishes a series of levers to be acted on by the muscles, for the purposes of motion, and a firm support for the soft parts of the system. The skeleton is divided into the head, trunk, and extremities; the head is again divided into cranium and face; the trunk is divided into spine, thorax, and pelvis; the extremities into the upper and lower, the upper consisting of arm, fore-arm, and hand; the lower of thigh, leg, and foot. The bones are divided into "flat bones" (fig. 74) and "long bones" (fig. 75), the flat bones (as a general rule) form the boundaries of the cavities, and the long bones the levers.

The bones consist of earthy matter, chiefly phosphate of lime, held together by a sort of cartilage, and arranged in fine scales or plates forming interstices or cells (fig. 76), these are large in the centre of the bone so as to make it nearly (and in some instances quite) hollow, and very small at the surface, so as to make it there nearly solid; this arrangement is found to be the very best to secure strength and lightness, together with a very slight degree of flexibility. The general form of a long bone is that of a shaft, having an enlargement at either extremity, the shaft is generally curved slightly so as to give elasticity, and is more solid than the ends, which are expanded to give size and firmness to the joints, and porous to keep them light; the shaft is small and close in its texture, that there may be plenty of room for the muscles which lie on and cover it. The skeleton then, consists of bony cases which enclose the viscera, a series of levers covered with, and moved by the muscles, and the whole forming a solid framework to support the soft parts. All the various motions of the body, from the winking of the eyelid and the wonderfully adjusted motions of the eye itself--which are so accurate, that a certain perceptible motion is required to look alternately at one side and the other of a pin's head--to the most powerful stroke of the arm, are performed by the contraction of the muscles. But there are several actions which would at first appear to be produced by elongation instead of contraction, such as the protrusion of the tongue, the winking of the eyelids, and the closure of the lips; but the first of these is produced by the contraction of a muscle which is attached at one extremity to the under part of the root of the tongue, and at the other to the inner part of the lower jaw, which, when it contracts, draws the whole tongue forwards, causing a part of it to protrude; the eyelids and lips are closed by circular muscles surrounding them, which, by contraction, draw together those parts.

The muscles form the flesh of an animal, or that part which is of a red colour, and which is called lean (in contra-distinction to fat), they are composed of fibres, each fibre made up of a number of fibrils, and the whole bound up together by means of a fine membrane, called areolar tissue. Each of these fibrils consists of a number of cells pressed closely together and having a peculiar bearded appearance (figs. 77, 78). It is by the sudden approximation and flattening of these cells that the muscle is shortened during contraction, but the contraction of each fibril is only for an instant, and then a relaxation and elongation of the cells takes place, which is followed by a second contraction, and so on. Now, the way in which a muscle keeps in a state of permanent contraction is this: the various fibrils or strings of cells relieve each other instead of all contracting and all relaxing together, and in this way a part only of the fibrils are in a state of contraction at any given time, while the others are relaxed, and as one begins to relax another contracts, and so keeps up a state of contraction in the muscle up to a certain point, when the contractions become gradually more feeble, and finally cease altogether; the muscle is then exhausted, but, if allowed to rest awhile, it again obtains its contractile power. This peculiar contractile power is a physical one, and exists for a short time after death, that is, until its perfect structure is altered by the beginning of decomposition; and what is a curious fact, the muscles of a dead body stimulated to contraction by pricking or galvanism, will tire and again obtain their contractility by rest, just as in the living body. The nature of the stimulus which causes the muscles to contract during life is not known; but, whatever it may be, it is conveyed from the spinal cord and brain, through the medium of a set of nerves, called the "motor nerves," or nerves of motion, to the muscles, to every fibre of which they are distributed. There is, however, another set of nerves which have nothing to do with motion, although exactly similar in appearance and generally associated and bound up in the same sheath with them; these are the nerves of sensation. They take their origin from every part possessed of sensibility or feeling--more especially the skin--and convey every impression of feeling to the brain. The muscles are not all under the control of the will, for the muscles of the heart, bowels, &c. are quite out of our control. Those which are subservient to our will are called "voluntary" muscles, or the "muscles of animal life," while those out of the control of the will, are the "involuntary" muscles, or "muscles of organic life" (fig. 79). These last have a structure different to the voluntary muscles, being composed of flattened fibres containing granules and overlapping each other. The tendons are the cord-like extremities of the muscles which connect them to the bones; they are fibrous and slightly elastic, flexible, and exceedingly strong; while moist they have a splendent appearance, somewhat like mother-of-pearl.

The ligaments are the strong fibrous bands which connect the bones together. Cartilages are substances of a white colour, smooth in their texture, and not very flexible; they are connected to the bones which they sometimes serve to prolong, and are themselves often converted into bone in old people, by the deposition of earthy matter within their structure, in fact, they may be considered as bone in an undeveloped state; for, in the infant, many parts are cartilaginous, which, in the adult, are bony; such as the ends of the long bones, the bones of the nose, &c.

With respect to the development of the form of the human body, there are two chief conditions which influence it, namely, age and sex. With respect to age, the proportional magnitudes of the different parts, early and late in life, vary very considerably. In the infant (fig. 80) the hand is by far the most completely developed (in size), next to this the abdomen, then the chest and upper extremities, and, finally, the lower extremities. In the adult (fig. 81), the largest measurement is round the chest and shoulders, but in the infant it is round the head; in the adult the lower extremities weigh one half of the whole body, but in the infant not one quarter. The infantile face has many peculiarities, so also has the face of old age (fig. 82). In the infant the lower part of the face is but little developed, the lower jaw is small, the chin scarcely at all prominent, and the distance from the nose to the chin very short, owing to the absence of the teeth, or (when formed) their smallness. The bones of the nose are scarcely formed, and this organ has therefore no bridge, properly so called; the nostrils are small, the lower part of the forehead small, smooth, and rounded, and the arches of the eyebrows but little prominent. The cheek-bones are small, and so are the bony arches which join them to the temples; the cheeks are full of fat, and the angles of the jaws rounded. In middle age the lower part of the forehead becomes more fully developed, the bony edges which support the eyebrows project and overhang the eyes, the bridge of the nose and the nostrils become more fully formed, the space from the nose to the chin is greater, the teeth cause the lips to be pushed more forward, the lower jaw is larger, and the cheek-bones show more plainly. In old age the whole upper part of the face becomes more marked, the nose and eyebrows still more prominent, the cheeks hollow from absorption of the fat, the space from the nose to the chin (as in infancy) shortens from loss of teeth, but the chin, being more fully formed, projects as the jaw rises to fill the space occupied by the teeth. The angles of the jaw are also very sharp and square, the eyes sunk in the head, and the whole skin of the face loosened and wrinkled from loss of substance beneath. With respect to sex, in the Male the facial bones are more fully formed, the shoulders broader and higher, the collar-bones longer and more curved, the chest wider, the hips narrower, and the legs longer, and every bone has its processes or markings more fully developed, and is more contorted from the action of the muscles, which are larger and more powerful than in the Female. In the Female (fig. 83) there is a greater deposition of fat, and, in many other respects, a tendency to a child-like conformation.

The body is everywhere covered by a common integument called the skin, beneath which is a layer of areolar and adipose tissue, which last has its cells filled with fat, which during life is in a liquid or oily state; this serves to protect the parts beneath both from cold and violence, fills up the hollows and irregularities between the muscles, and gives the body a more rounded and graceful appearance.

Man, in common with all the higher animals, is possessed of the five "special senses," namely, sight, smell, hearing, touch, and taste, the organs of which are the eyes, nose, ears, skin, and mouth; but these are developed in very different degrees in different animals.

Sight is the sense by which we take cognisance of those external objects which either give out or reflect light. The eye (fig. 84) is an optical instrument of great beauty and perfection, and by its help an image of everything we look at is produced in miniature on the expanded optic nerve called the "retina," as an image is produced in the "camera," which instrument is made in imitation of the eye; this image by some unknown power is perceived by the mind, and produces the sense of vision. By the sense of sight we recognise all forms, colours, and gradations of light and shade, and also (assisted by the experience gained by the sense of touch) the qualities of surface, such as roughness, smoothness, &c.; for instance, we know by touch that a piece of glass is smooth, that a piece of velvet has a different kind of smoothness, &c., and the mode in which these surfaces reflect the light, teaches us to recognise them so that we could predict from touch what appearance an object would have, or by the look how it would feel. Our knowledge of the solidity of bodies is known to the sense of sight by (unconsciously) looking at them with each eye alternately, so as to see to a certain extent round them, or somewhat on either side alternately. By this fact the principle of the stereoscope is explained, the pictures looked at through it, consist of two views of the same object, taken at points of sight a short distance apart (corresponding to the distance between the eyes) and they give us, when steadily looked at, the same result that real objects do when looked at with each eye, alternately. It is a curious fact, but one which has been clearly demonstrated by optical experiments, that the picture represented on the "retina" is inverted, but from habit we refer things to their natural position, and how naturally and easily this is done, may be ascertained by holding the head down close to the floor, and looking at objects through one's legs, in this position they are all inverted; but we naturally make ourselves satisfied that they are still as they were before, so that they really do not appear to be inverted. The impression formed by objects upon the retina is not immediately destroyed upon their removal, but remains for an instant permanent; this gives rise to a great many curious phenomena, among which the most familiar is the circle of light produced by the rapid evolution of any luminous body, as the red-hot end of a stick. This is also shown by a toy, called the "Thaumatrope," which consists of a piece of card having a small wire attached to the upper part by which it may be turned rapidly round by the thumb and finger. On one side of the card is painted an object, such as a parrot, and on the other a cage; when the card is turned round, first the bird and then the cage is presented to the eye, but the image of the one has not left the eye before the other is presented to it, and the result is the uniform appearance on both sides of a parrot in a cage.

The structure of the eyes in Man and the higher animals is very nearly alike, so that the same description will serve for both, but those of insects and the lower orders of animated beings are very different.

A. Sclerotic. B. Choroid. C. Retina. D. Vitreous humour. E. Aqueous humour. F. Cornea. G. Crystalline lens. H. Pupil. I. Optic nerve. ]

The eye consists of various humours of different densities enclosed in membranes (fig. 85); the human eye is spherical for the posterior four-fifths of its circumference, but the front part projects a little, being formed of part of a smaller sphere. The globe of the eye, all but this front part, is enclosed in a firm fibrous coat called the "sclerotic," within which is a membrane full of blood-vessels called the "choroid," this on its inner side is covered with a black substance called the "black pigment," and within this (lining the ball) is the nervous coat or retina, it is the optic nerve so expanded as to form a lining to the membranes of the ball of the eye for the greater part of its extent. The ball of the eye is filled with a transparent fluid, somewhat glutinous, called the "vitreous humour;" stretched across the front part of the ball of the eye is a membrane having a round hole in its centre, called the "iris," coated (like the choroid) with black pigment on its back surface; this membrane (the iris) is the coloured part of the eye, and its transparency or opacity determines the colour of the eye. The pupil is the hole in its centre, it looks black because the black pigment which covers the inside of the ball of the eye (for the retina is transparent) is seen through it; behind the iris and in front of the vitreous humour, is the crystalline lens, it is just the shape of a very thick lens, but is more convex behind than in front, it is quite transparent, and of a density almost approaching to solidity. The front fifth of the ball of the eye is covered in by a dense transparent substance called the cornea, of the form of a watch-glass and quite transparent, between this and the crystalline lens there is a small space called the anterior chamber, filled with a clear watery fluid called the aqueous humour. The eye is moved by six muscles, four straight and two oblique, these turn it in every direction with the greatest accuracy; the front of the eye is defended by the eyelids, which by winking and being moistened by the tears, keep the eye always moist and free from dust.

The eyes of insects and several other classes of animals are called compound eyes, and the image they produce must resemble objects depicted in mosaic, or made up of small spots of distinct colours; they consist of hundreds of small tubes, radiating and forming part of a sphere on each side of the head, each of these tubes admits the light from objects at which they point. Some of the animals still lower in the scale have simple spots of transparent membrane with an expansion of optic nerve beneath them; in such creatures the power of vision is probably limited to distinguishing light from darkness.

a, external opening; b, tympanum; c, small bones; d, Eustachian tube; e, cochlea; f, semicircular canals.]

Hearing.--Sound is conveyed to the mind by the vibration of air, and this vibration is communicated to a membrane within the ear, called the "tympanum;" this is in connection (by means of three minute bones) with the expanded auditory nerve, which ramifies within the "vestibule," which is filled with a fluid, so that the vibrations of the air are thus communicated to the nerve of hearing (fig. 86).

Smell.--The interior of the nose is lined by a fine moist membrane, on which the olfactory nerve is distributed. The cause of odour is quite unknown; though, as most volatile substances are odorous, it is supposed to be some volatile matter which comes in contact with the olfactory nerve, or nerve of smelling. The quantity given off in some instances must be exceedingly small, for one grain of musk placed on a piece of glass will give out scent for months, and at the end of that time weigh still one grain. Some animals, as Dogs and Birds of prey, have this sense so acute that it serves them instead of almost every other; the Vulture will scent carrion miles away, and the Dog will track his master by the power of scent.

Taste.--The sense of taste resides chiefly in the tongue, but to some extent in the palate or roof of the mouth; it appears to be a modified sense of smell, combined with common sensation or touch, and is much diminished by loss of smell.

Touch, next to sight, is the most important of the senses. The whole skin possesses this sense, but in some parts far more than others, the finger-ends being the most perfectly endowed with it; and in the lower animals the parts which are used as prehensile organs, are in them the most acutely sensitive to touch, as the end of the proboscis in the Elephant, and the lips of most other quadrupeds. In the Feline tribe, the smellers or hairs from the side of the face, appear to be true organs of touch, and enable them to feel their way in the dark. By the sense of touch we are informed of the amount of resistance of matter (and consequently its weight), of hardness, smoothness, roughness, and other qualities of surface, and by the amount of extension of the hand to reach them of their distance from us, and also of the form and size of things.

The human hand is the most perfect organ of prehension possessed by any animal, for although Monkeys have hands, yet these are so imperfect as to serve chiefly for clinging to, and climbing amongst the branches, they cannot take any small substance between the thumb and finger, while the human hand, besides enabling its possessor to grasp firmly with all the fingers and thumb, each can individually be approximated to the thumb so as to hold the very smallest substance, and as Man is so constituted that he can stand firmly on his feet, his hands can be solely appropriated to the acts of prehension. It has been said that Man owes much of his superiority to the lower animals, to the possession of hands which can take, examine, and compare everything within his reach; but it seems more probable that these hands have been given him because he, only, has the faculties of examining and comparing. The Lion and Tiger would be no better off, had they Man's hands instead of their feet with claws--probably much worse.