PART VI.

ANIMAL PHYSICS.

SECTION I.-ANIMAL MECHANICS.

ANALYSIS OF THE SECTION.

Animal Mechanics.-Proportion of solids and liquids in the human body. · The large proportion of water in the structures of the body.—The properties of the organs dependent on water.-Relative weight of mineral and organic matter in the body. The mechanism of the skeleton.—The skull.—Ossifica tion.-Structure of the teeth.-Serpent's tooth.—The spine or back-bone.—Its form, flexibility, and strength.-The ribs and their muscles.-The shoulder joint. The bones of the arm.-Use of ligaments.-The hip-joint.-Thighbone.-The knee-joint and legs.—The arch of the foot.—Effects of artificial pressure. Muscular force.—Its exhaustion. Composition of bone.

Animal Mechanics---Mechanism of the Human Skeleton. 1073. HAVING in a former part of the volume reviewed the doctrines of general mechanics, we can proceed, with the light thence derived, to consider the solid framework of the human body, a perfect structure which Divine Wisdom has willed into existence to serve the purposes of life and happiness to man.

There is scarcely a part of the animal body, or an action which it performs, or an accident that can befall it, or a piece of professional assistance which can be given to it, that has not a close relation to the truths of natural philosophy as here set forth. Three centuries have not yet passed since the renowned Dr. Harvey made the most important discovery in regard to the nature of living beings which anywhere had yet been made, namely, that of the never-ceasing circulation of the blood in every part of the animal frame.

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Constituents of the Human Body.

1074. The human body is composed of solids and liquids, the former represented by the bones, the flesh, and soft organs; the latter chiefly by the blood. We should form a very inadequate notion of its constitution, if we restricted the liquid portion to the blood, the average amount of which, in the adult body, is estimated at thirty pounds. At least two-thirds of the weight of the human body are represented by water. Water is present even in the bones; but if we except these, three-fifths of the flesh and solid organs, and about four-fifths of the liquid constituents, consist of water. The flexibility, softness, elasticity, tenacity, and other physical properties of the soft organs and muscles, are due to the presence of water. A dry muscle could not contract; and ordinary skin, which is highly flexible and elastic, loses these physical properties and becomes hard, dry, and brittle, when deprived of water. Without this abundance of water in the organs, the living powers of the body could not be exercised. These remarks equally apply to the whole animal kingdom. An oyster contains 81 per cent. of water, and there are some small jelly-fish (acalephæ) which contain as much as 99 per cent. of this liquid, so that they might be almost described as living water. From calculations based on the chemical analysis of bone and of the solid and fluid constituents of the body of an adult, weighing 150 pounds, it appears that 100 pounds consist entirely of water, and the residuary weight is thus made up,-of dry organic matter, 34 pounds; earthy or mineral matter, chiefly phosphate and carbonate of lime and common salt, mixed with small quantities of earthy and other salts, and oxide of iron, 9 pounds; and of oil or fat, 7 pounds. It is probable that the proportion of water is even greater than that which is here assigned; as it is, it amounts in an adult to ten gallons!

1075. In reference to the skeleton, the cranium or skull has been already mentioned as an instance of the arched form answering the purpose of giving strength. The brain, in its nature, is so delicate or susceptible of injury, that slight local pressure disturbs its action. Hence a solid covering like the skull was required, with those parts of it made stronger and thicker which are most exposed to injury. An architectural dome is constructed to resist one kind of force only, always acting in one direction, namely gravity, and therefore its strength increases regularly towards the bottom, where the weight and horizontal thrust of the whole are to be resisted; but in the skull, as in a barrel or egg-shell, the mere tenacity of the substance is many times greater than that which is sufficient to resist

The Skull. Ossification.

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gravity, and therefore its peculiar form is calculated to resist forces of other kinds, operating in all directions. When we reflect on the strength displayed by the arched film of an egg-shell, we need not wonder at the severity of pressure, and even of blows, which the cranium can withstand.

In the early fœtal state, that which afterwards becomes the strong bony case of the brain, exists only as a tough flexible membrane. Ossification commences in this membrane long before birth, at a certain number of points from which it spreads, and the portions of the skull formed around these points, soon acquire the appearance of so many scales or shells applied on the surface of the brain, and held together by the remaining membrane not yet ossified. These afterwards become firmly fixed together, by projections of bone from the edges of each, shooting in among similar projections of the adjoining ones, until all mutually cohere by perfect dovetailed joints, as does the work of a carpenter. These joints are called the sutures of the cranium, and are visible to extreme old age. Through early childhood, the cranium remains to a certain degree yielding and elastic, causing the falls and blows, so frequent during the lessons of walking, to be borne with comparative impunity.

A severe blow on a narrow part of the skull, as the blow of a hammer, generally fractures and depresses the part struck; while one less severe, but with more extended contact, being stoutly resisted by the arched form, often injures the skull by what corresponds to the horizontal thrust in a bridge, causing a crack at a distance from the place struck, generally half-way round to the opposite side. This kind of fracture is well known to medical men under the name of counter-stroke. Thus it often happens that a violent blow on the summit of the head causes a fatal fracture through the base of the skull.

1076. In the lower jaw we have to remark the greater mechanical advantage, or lever-power, with which the muscles act, than in other parts of animals. The temporal muscles pull almost directly across, or at right angles to the line of the jaw, while in most other cases, as in that of the muscles about the shoulder-joint lifting the arm, the muscles act very obliquely, and with power diminished in proportion to the obliquity. Even the human jaw can crush a very resisting body; and the jaws of the lion, tiger, shark, or crocodile, have an astonishing power of crushing substances.

The teeth rank high among those parts of the animal body which appear almost as if they were the results of distinct miracu

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Construction of the Teeth.

lous agencies constantly renewed, so difficult is it to suppose simple laws of life capable of producing the variety of form and fitness, constantly changing with age, which they exhibit. They constitute a beautiful set of chisels and wedges, so arranged as to be most efficient for cutting, tearing, and grinding the food, with an exterior enamel so hard that few substances in nature can make any impression upon it. In the Rodentia, or gnawing animals, such as the beaver, rat, and squirrel, the front of the tooth is formed of a layer of hard enamel, while the back part consists of dentine or osseous matter. As this is worn away more rapidly by friction than the enamel, the edge of the tooth is always kept sharp. In early states of human society, teeth were used for many purposes for which steel is used now. It seems, however, as if the laws of life, marvellous to human intellect as they are, had still been inadequate to cause teeth, cased in their hard polished enamel, to expand or grow as the softer bones grow; and hence has arisen a provision more extraordinary still. A set of small teeth come soon after birth, and serve the child until six or seven years of age: these then fall out, and are replaced by larger ones, which endure for life; the number of the latter being completed only when the man or woman is full grown by the four teeth, called wisdom teeth, then appearing to fill up the more spacious jaw.

Nothing can be firmer than the setting of the fangs of the teeth in the bony cavities of the jaw. It resembles that of a nail driven into a board, and is known under the name of gomphosis (yóμpos, a nail). The teeth have to sustain great force during mastication, and by this mode of immovable attachment the pressure is equalized. The teeth of all animals are admirably adapted by form and structure for their intended uses. The poison teeth or fangs of the rattle-snake and other venomous serpents are in this respect re

markable. The tooth is curved (see fig. 324), and is grooved or channelled on the front or convex side, as if it were folded upon itself. It is through this channel that the poison is ejected into the deep and curved puncture, made by the bite through the skin. The point of the tooth is solid, intensely hard, and finely sharpened. It consists of pure enamel. The channel or groove through which the poison is discharged terminates in the front of the tooth at a short distance above the point (a, fig. 324). By this arrangement its sharpness is preserved. The

Fig. 324.

The Spinal Column.

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base of the tooth is on a movable joint, so that it lies back in the upper jaw until the animal uses it, when the act of bringing it forward, jerks the poison through it. There are several rudimentary

fangs ready to supply the place of this, if broken.

The spine or backbone, in its structure, has as much of beautiful mechanism as any part of our complex frame.* It is the central pillar of support and great connecting medium of the other parts. It has, at the same time, the office of containing within itself, and of protecting from external injury, a prolongation of the brain, called the spinal marrow, more important to mere animal life, than the greater part of the brain itself. It thus unites in itself the apparent incompatibilities of great elasticity, great flexibility in all directions, and great strength, both to support a load and to defend its important contents.

1077. The head rests on the elastic column of the spine as softly as the body of a carriage rests upon its springs. Between each two of the twenty-four vertebræ, or distinct bones of which the spine consists, there is a soft elastic intervertebral substance, about half as bulky as a vertebra, and which yields readily to any sudden jar. Then the spine has a waved or bent form somewhat like an italic f, as is perceived on viewing it sideways, or in profile, and owing to this, also, it yields to any sudden pressure operating against either end. The bending might seem a defect in a column intended to support weight; but the disposition of the muscles around, is such as to leave all the elasticity of that form, and a roomy thorax or chest, without any diminution of strength. It comes forward in the neck to support the skull it recedes in the chest to allow space for the lungs, and it again advances to support the viscera of the abdomen.

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The spine has been compared to a chain, because it consists of twenty-four distinct pieces or bones. They are kept in contact by smooth rubbing surfaces, which allow of a degree of motion in all directions; and a little motion comparatively between two adjoining pieces, becomes a great extent of motion in the whole line of vertebræ. The strength of the spine as a whole, is shown in the fact that a man can carry upon his head or back a weight heavier than himself; and the strength of each separate vertebra surrounding the

* The presence or absence of a spine creates two great divisions of the animal kingdom: 1, the Vertebrata, including man and the higher classes of animals, which are endowed with this structure, and with it a brain and spinal marrow; and, 2, the vertebrata, which are destitute of a spine.