Ball-and-socket Joint.
We will now see how some of the most useful mechanical inventions have had their prototypes in Nature.
There is, for example, the well-known “Ball-and-socket joint,” without which many of our instruments, especially those devoted to optical purposes, would be impracticable.
The figure on the right hand of the illustration represents the “bull’s-eye” of my own microscope. It will be seen that there is a ball half sunk in a cup, so that it can be turned in any direction. In point of fact, the upper part of the ball is nearly concealed by another cup, but, in order to show the structure, the upper cup has been removed. Who was the inventor of the ball-and-socket joint I do not know, but I have little doubt that he must have had in his mind many natural examples of this joint, three of which are represented in the illustration.
On the left hand are seen the upper part of the human thigh-bone and that part of the hip-bone into which it fits.
The reader will see that at its upper end the bone takes rather a sharp turn, and is then modified into a ball. This ball fits into a corresponding socket, technically named the “acetabulum,” and is thereby endowed with freedom of motion in almost every direction. Generally we do not practise our limbs sufficiently to develop that full freedom, but those who have seen any good professional acrobats must have been struck with the wonderful mobility of which the human body is capable.
The socket is not a deep one, but dislocation of the hip is exceedingly rare, the bone being held in its place by three powers. The first is due to a short ligament, which, however, does not always exist, but, when it is present, is useful in retaining the bone in its place. Then there is the contractile power of the thigh muscles, which are always forcing the ball into the socket. Lastly, there is the pressure of the atmosphere, a force which is seldom taken into consideration, but which has great influence on many parts of the human frame. This part of the subject will be resumed when we come to treat of Atmospheric Pressure.
The arms are jointed to the shoulder-blades in a very similar manner, the upper arm-bone, or “humerus,” being furnished with a rounded end, and fitting into a cup-like cavity in the shoulder-blade, or “scapula.” This formation can easily be seen by separating the different bones of a shoulder of mutton.
At the bottom of the illustration are given two vertebræ of a snake, separated in order to show their structure. It will be seen that each joint has a ball in front and a socket behind, thus giving the creature that wonderful flexibility which is quite proverbial, and without which it could not seize its prey.
The following eloquent passage is taken from Professor Owen’s work entitled “The Skeleton and the Teeth:”—
“Serpents have been regarded as animals degraded from a higher type, but their whole organization, and especially their bony structure, demonstrate that their parts are as exquisitely adjusted to the form of their whole, and to their habits and sphere of life, as is the organization of any animal which we call superior to them.
“It is true that the serpent has no limbs, yet it can outclimb the monkey, outswim the fish, outleap the Jerboa, and, suddenly loosening the coils of its crouching spiral, it can spring into the air and seize the bird upon the wing: all these creatures have been observed to fall its prey.
“The serpent has neither hands nor talons, yet it can outwrestle the athlete, and crush the tiger in the embrace of its ponderous overlapping folds. Instead of licking up its food as it glides along, the serpent uplifts its crushed prey, and presents it, grasped in the death-coil as in hand, to its slimy, gaping mouth.
“It is truly wonderful to see the work of hands, feet, and fins performed by a modification of the vertebral column—by a multiplication of its segments with mobility of its ribs. But the vertebræ are especially modified, as we have seen, to compensate, by the strength of their numerous articulations, for the weakness of their manifold repetition, and the consequent elongation of the slender column.
“As serpents move chiefly on the surface of the earth, their danger is greatest from pressure and blows from above; all the joints are fashioned accordingly to resist yielding, and sustain pressure in a vertical direction; there is no natural undulation of the body upwards and downwards—it is permitted only from side to side. So closely and compactly do the ten pairs of joints between each of the two hundred or three hundred vertebræ fit together, that even in the relaxed and dead state the body cannot be twisted except in a series of side coils.”
The upper right-hand figure represents a portion of the shell of an Echinus, or Sea-urchin, together with two of the spikes.
The reader will remember that in the description of the Heart-urchin, and the mode in which it dug its way into the sand, the peculiar mobility of the spines was mentioned. How that mobility is produced we shall now see.
If a living Sea-urchin can be procured, and placed in a glass vessel filled with sea-water, it will at once be seen that its surface is thickly covered with spines. In some species these spines are as thick as ordinary drawing pencils; but in most of those which are found on our shores they are very slight, and scarcely longer than darning-needles. They are in almost perpetual motion, and generally have a sort of revolving movement, the base being the pivot.
Now, if we take a dried shell of the Sea-urchin, we shall find that the spines will come off with a touch, and, indeed, to preserve one with all the spines complete is a most difficult business. Let us, therefore, pull one from its attachment, and examine its base. This will be found to be swollen into a cup-like form, as seen in the illustration; and, if we look at the spot whence it came, we shall see that there is a little, rounded, polished prominence, exactly fitting into the cup, just as the ball of the human thigh-bone fits into the acetabulum. It has also its ligament to keep it in its place, and its same set of muscles that move it, and is altogether a most wonderful piece of mechanism. There are in some species of Echinus about four thousand of these spines.
The legs of an insect afford excellent examples of the ball-and-socket principle, the socket being on the body, and the ball on the base of the leg. Some of our largest insects—such, for example, as the common Stag-beetle—exhibit this principle very well. I have now before me a Stag-beetle which has been dead for many years, and is quite dry and hard. Yet I can rotate the legs almost as freely as if the beetle had been just killed, so easily do the joints work. Even the antennæ, which are affixed to the head by a similar joint, move about by their own weight on merely changing the position of the insect.
These are only a few of the many natural examples of the Ball-and-socket joint, but they are sufficient for our purpose.