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In some animals the bony structure is on the outside of the body, as in all the testaceous tribes, which are enclosed in one or more shells ; as the oyster, snail, whilk, &c.; and also in the crustacea, which comprise the crab, lobster, shrimp, &c.
In the crustaceous, as well as in the testaceous, there is a power of renewing the shell in case of injury, which in the former, not only extends to the shell, but likewise to the limb itself. Lobsters and crabs, are sometimes, after thunder-storms, found to be entirely without their claws, which require some time for reproduction. The jar communicated to the water, and perhaps terror on the part of the animal, have the singular effect of making these animals throw off their claws. The effect seems to be voluntary, for some of the younger of these animals will drop their claws, on an attempt to take them, even though they have not been touched. In these animals, the blood-vessels have the power of secreting the matter of the shell. Crabs and lobsters lose their shell annually, and seek retirement till the new shell is sufficiently consolidated; being aware of their defenceless state at such times.
MARKS OF DESIGN IN THE HUMAN BODY.
I challenge any man to produce, in the joints and pivots of the most complicated, or the most flexible machine that was ever contrived, a construction more artificial, or more evidently artificial, than that which is seen in the vertebræ of the human neck. Two things were to be done. The head was to have the
of bending forward and backward ; and, at the same time of turning itself round upon the body to a certain extent. For these purposes, two distinct contrivances are employed. First, the head rests immediately upon the uppermost part of the vertebræ, and is united to it by a hinge-joint ; upon which joint, the head plays
freely forward and backward. But then the rotatory motion is unprovided for ; therefore, secondly, to make the head capable of this, a farther mechanism is introduced ; not between the head and the uppermost bone of the neck, where the hinge is, but between that bone and the next underneath it. This second, or uppermost bone but one, has what anatomists call a process, viz. a projection, somewhat similar in size and shape to a tooth ; which tooth entering a corresponding hole or socket in the bone above it, forms a pivot or axle upon which that upper bone, together with the head which it supports, turns freely in a circle. Thus are both motions perfect without interfering with each other. When we nod the head, we use the hinge-joint, which lies between the head and the first bone of the neck : when we turn the head round, we use the tennon and mortice, which runs between the first bone of the neck and the second. No one can here doubt of the existence of counsel and design.
The spine, or back-bone, is a chain of joints of very wonderful construction. It was to be firm, yet flexible : firm, to support the erect position of the body; flexible, to allow of the bending of the trunk in all degrees of curvature. It was further also to be a pipe for the safe conveyance from the brain, of the spinal marrow; a substance not only of the first necessity to action, if not to life, but of a nature so delicate and tender, so susceptible of injury, as that any unusual pressure upon it is followed by paralysis, or death. Now the spine was not only to furnish the main trunk for the passage of the medullary substance from the brain, but to give out, in the course of its progress, small branches, which being afterwards indefinitely subdivided, might, under the name of nerves, give, to every part of the body, the
power of feeling and motion. The same spine was also to serve another purpose, not less wanted than the preceding, viz. to afford a basis for the insertion of the muscles, which are spread over the trunk of the body; in which trunk there are not, as in the limbs, cylindrical bones to which they can be fastened. The spine had likewise to furnish a support for the ends of the ribs to
How admirably is all this accomplished! The spine is composed of a great number of bones, (in man, of twenty-four,) joined to one another, and compacted by broad bases. The breadth of the bases upon which the parts severally rest, and the closeness of the junction, give to the chain its firmness and stability, the number of parts, and consequent frequency of joints, its flexibility. This flexibility varies in different parts of the chain; is least in the back, where strength more than flexure is wanted ; greater in the loins, which it was necessary should be more supple than the back ; and greatest of all in the neck, for the free motion of the head. In order to afford a passage for the descent of the spinal marrow, each of these bones is bored through in the middle in such a manner, as that, when put together, the hole in one bone falls into a line, and corresponds with the holes in the two bones contiguous to it. By which means, the perforated pieces, when joined, form an entire, close, uninterrupted channel ; at least, while the spine is upright, and at rest. a settled posture is inconsistent with its use, a great difficulty still remained, which was, to prevent the . vertebræ shifting upon one another, so as to break the line of the canal as often as the body moves or twists. But the vertebræ, by means of their processes and projections, and of the articulations which some of them form with one another at their extremities, are so locked in and confined as to maintain, in the surfaces of the bones, the relative position nearly unaltered ; and to throw the change and pressure produced by flexion, almost entirely upon the intervening cartilages, or gristle, the springiness and yielding nature of whose substance admits of all the motion which is necessary, without any chasm being produced by a separation of the parts. For the medullary canal giving out in its course a supply of nerves to different parts of the body, notches are made on the upper and lower edge of each vertebræ ; two on each edge. When the vertebræ are put together, these notches, exactly fitting, form small holes through which the nerves issue out in pairs, in order to send their branches through every part of the
body, and with an equal bounty to both sides of the body.
The structure of the spine is not in general different, in different animals. In the serpent tribe, however, it is considerably varied ; but with strict reference to the convenience of the animal. For, whereas in quadrupeds the number of vertebræ is from thirty to forty, in the serpent, it is nearly one hundred and fifty : whereas in men and quadrupeds the surfaces of the bones are flat, and these flat surfaces laid one against the other, and bound tight by sinews; in the serpent, the bones play one within the other, like a ball and socket, so that they have a free motion upon one another in every direction ; that is to say, in men and quadrupeds, firmness is more consulted; in serpents, pliancy.
PALEY's Nat. Theology.
The muscles are distinct portions of flesh, capable of contraction and relaxation. They are composed of fibres of two kinds ; the one soft and irritable, of a red colour, from the blood that is in them : these generally constitute the body of the muscle; whilst the other sort are found, for the most part, in the extremities, and are of a harder texture, and of a white glistening colour: if these are formed into a round, slender cord, they are called tendons. What we commonly term flesh, as the lean of meat, is the substance of the muscles. The fibres of which they are composed are exquisitely fine.
The muscles are generally attached to the bones, by means of tendons, and are so artfully situated, that whatever motion the joint annexed is capable of performing, the muscle is adapted to produce it. The knee, and the elbow, furnish examples of this agreement. Both being hinge joints, formed to move backwards or forwards, the muscles belonging to them are placed parallel to the bone, so as, by their contrace
tion or relaxation, to effect that motion, and no other. The shoulder and the hip joints, by their construction, admit a sort of sweeping or circular action, and are accordingly supplied with muscles adapted to it.
A joint unfurnished with suitable muscles would be motionless ; muscles deprived of the joint, would be unavailing. They are necessary to each other ; and their union displays the highest marks of wisdom and goodness.
The red colour of the muscular or fleshy parts of animals is owing to innumerable blood vessels, that are dispersed through their substance. When we soak the fibres of a muscle in water, it becomes white. The blood vessels are accompanied by nerves ; and they are both distributed so abundantly in the fleshy parts, that in endeavouring to trace the course of the blood vessels in a muscle, the muscle would appear to be formed altogether by their ramifications; and in an attempt to follow the branches of its nerves, they would be found to be equally numerous.
When a muscle is in action, the fibres become shorter, and the body swells. Experiments show that the nerves, and a regular supply of blood, are essential to this contraction; and that it is regulated by the mind, at least in the voluntary muscles, viz. those muscles that move the limbs, or any other part dependently upon our will: but there are others, called the involuntary muscles, which operate without even our conscious
ness of the action that is continually going on within - us; such is the heart, which is itself a muscle : and the
muscular fibres that occasion the necessary motions of the stomach and the intestines.
Most muscles have others opposed to them, which act in a contrary direction, and are called antagonists. Some of these act in succession, as when one muscle, or one set of muscles, bends a limb, another extends it; one elevates a part, another depresses it; one draws it to the right, another to the left. By these opposite powers the part may be kept in a middle direction, ready to obey when called to act.
Four hundred and forty-six muscles have been described, and their uses ascertained. It has been said