Fall 2022 Final Exam Solution on the Skeleton Bones from the Chicago University
How Many Bones Are In The Human Body, And How Does Each Bone Function?
The traditional complement of bones in each human being is 206, but this is a general rule, not a law. For example, about one person in twenty has a thirteenth pair of ribs, and Mongols frequently have only eleven pairs. A baby is born with about 350 bones, some of which fuse in later life, some of which retain their identity throughout life. All bone fusion is over by the end of the growth period, perhaps in the twenty-fifth year for men and younger for women. An older adult will, accidents apart, retain their mature complement of bones, but many-notably women will have lost half of their bone content by the time seventy is reached. Bones not only provide 'skeletal support for muscles but perform other roles. There is even an argument that the structural and protective tasks of the skeleton should be considered secondary to the more important bone duty of providing a mineral reservoir. It is hard contemplating a blancmange of a human being, totally without a rigid framework. However, it is harder still to imagine the proper function of metabolism without extra supplies of calcium and phosphorus always readily available.
A total of 206 is achieved more from the limbs than from the main axial skeleton of the trunk and head. Each arm has thirty-two bones: one collar bone, one shoulder blade, one humerus, one radius and one ulna; eight wrist bones in two rows; five metacarpals in the palm, and fourteen phalanges, three to each finger and two to the thumb. Each leg has only thirty-one bones; being one hip bone, one femur, one kneecap, one tibia and one fibula, seven tarsals in the instep and heel, five metatarsals in the foot, and fourteen phalanges, three to each ordinary toe and two to the biggest toe, the hallux. With two legs and arms, this means a total of 126 bones. The axial skeleton has eighty. There are twenty-nine in the skull, of which eight are in the Cranium, fourteen are six in both ears as the ear ossicles, and one - the hyoid bone - is in the throat between the skull, Toure lower jaw and the upper larynx. The spine has twenty-six bones, of which, in descending order, seven are cervical vertebrae, twelve are thoracic vertebrae, five are lumbar vertebrae, one is the sacrum, and one is the coccyx or tail. The chest has twenty-five bones, of which one is the breastbone and twenty-four are ribs.
Unlike all other apes (as was mentioned in the chapter on the male), the human species has no bone in its penis; it has to make do without such an aid. Also, man can acquire bone during life according to his mode of existence. Cavalrymen have acquired bones in their buttocks and thighs, quite distinct and separate from the traditional hip and femur bones. As such extra bones have arisen entirely as a result of the cavalry- man's continual horse-work; this makes one wonder again about humanity's lack of an os penis.
The mammals, despite unevenness in size, ranging as they do from shrews to whales, are remarkably consistent in their bony skeleton. The quaintest example is the system of cervical vertebrae. These bones are quite distinct from the thoracic vertebrae which follow them, and yet the long-necked giraffe only has seven, and the no-necked whale also has seven. Exceptions to this rule of seven are the manatees and the tree sloths. The horse, able to transport a man easily, has 205 bones against 206 for the man. The horse has eighteen pairs of ribs (against the human twelve) and fifty-four bones in the backbone (against twenty-six); but, because the manner of its evolution forced the modern horse to be prancing around on its middle fingers and middle toes, there has been considerable bone loss; horse limbs have only twenty bones each, against thirty-two and thirty-one for human limbs.
As everyone knows but tends to disregard, the fact that two children are the same chronological age is relevant but frequently misleading. Neither intellectual nor sexual maturation pays such diligent regard to the calendar as humans do. Skeletal maturity is equally casual about birthdays, but its degree of development can be of terrifying importance to some children. Does this girl's above-average size mean that she will end up taller than average and, therefore, useless for ballet? Does this girl's current height indicate that she may be over 6 ft. (1.8 m.) when eighteen, and should she try to slow down her growth while there is still time? An X-ray of the wrist will probably answer. Bone development follows a definite routine, and the stage reached indicates the proximity of the final stage, the end of growth.
Sex differences in adult size and shape come about in different ways. The longer male legs result in, the more time boys spend growing before puberty when legs grow faster than the trunk. Conversely, the longer male forearm is established at birth and continues to be longer than the female forearm throughout life. Similarly, the female tendency to have a longer second finger (index) than the fourth finger (ring) exists from birth. Yet another cause of sex differential is a direct intervention by male and female hormones. This happens both in the shoulders (broader for men) and in the hips (broader for women) when androgens and oestrogens stimulate cartilage growth.
Bone growth and form are undoubtedly genetic, but the demands on it influence it. Unlike some structural girders, it is altered and improved, provided the load is insufficient to cause damage by any increased demands being made upon it. Both the volume and density of bone can be made to increase my work. For example, two groups of growing rats were fed differently. One group received hard food that needed chewing, and the other received soft food that did not. Not only did the soft-fed rats acquire slightly smaller heads and faces- by 1-2%, but the bones of their head and jaw were lighter - by 12%. One wonders whether the steaks of America help produce the thick-set jawline characteristic of the well-fed American citizen or if it is just good feeding.
Mice have been kept in centrifuges at four times the force of gravity and have thicker and differently shaped bones. Human beings in bed promptly lose bone, not just de-calcification but the bone itself, which is lost when the skeleton is abnormally idle. For the same reason, the sudden change and call-up from a sedentary life to an Army battle course can lead to certain types of fractures. Demands are being made of a skeleton unprepared for such relentless effort. It is amazing that astronauts, particularly the Soviet variety who have spent many months continuously in orbit, have been able to walk within a short time (apparently less than twenty-four hours) of returning to earth's gravitational pull after so long in the weightlessness of space.
The bone grows to a large extent where it ought to grow, where the stress demand that it should be grown. Should a child break a femur, and should that bone fracture be wrongly aligned, the healing stresses will look initially unsightly as a large callus is formed, but the fracture site ought to be invisible a couple of years after the break. Bone construction cells will have combined with bone destruction cells to join the break and erase the broken outline of the faulty alignment. Unfortunately, the repair system grows increasingly inefficient with age, and faulty alignment is more likely to lead to permanent thickening.
The word skeleton is derived from a Greek word meaning dried up. Bones are most frequently seen when dried up, but their living strength depends upon their not being dry. Chemically, bone tissue is 70% inorganic and 30% organic. Dissolve away much of the inorganic component with an acid, and the result is like a dog's rubber bone; in fact, a long bone like the human femur can be tied in a knot. Remove the organic component, either by burning or by letting decay seek it out, and the result will be a dry, brittle, hard object with more of the properties of cast iron than the flexible and stronger steel of the living tissue.
Although the body's 206 bones are long (like the femur), short (as in the wrist), flat (like the shoulder blade) or irregular (like the vertebrae), all bones have an outer, denser layer of compact bone and an inner meshwork of porous material. The outer layer benefits from the principles which cause a tube to be almost as strong as a solid rod of the same diameter. The inner layer, often called spongy because it looks like that, is phenomenally strong. More like hard coral than sponge, its strength also follows the engineering principles which explain why many meshworks, such as honeycomb, can be so strong and yet apparently so delicate.
Both inner and outer strength is necessary. When a parachutist touches down or when someone jumps from a wall, the traditional loads are exaggerated many times. The leg bones are capable of withstanding compression of a ton or more. Later, particularly in post-menopausal women, bones can be unfortunately vulnerable to far smaller loads, and the neck of the femur is a frequent victim of quite modest accidental burdens imposed upon it.
The function of bone is diverse. Its rigidity provides shape and support. It protects. It acts as an anchor point for muscles. Its internal marrow manufactures all red blood corpuscles. It also produces other constituents of blood, as well as destroys old red cells. It contains minerals, chiefly calcium and phosphorus, magnesium, fluorine, chlorine, and iron. More important than their mere presence, the bone's minerals are frequently removed and replaced. For all its structural solidity, bone is remarkably changeable.
In the earliest fishes, the skeleton was external, much like the armour of a mediaeval knight. Not only is such external plating a restriction to movement, but it can have little to do with the body's metabolism. Like teeth, it was probably deposited in a one-way process; once deposited, the minerals were as good as lost. The calcium carbonate and calcium phosphate of bone can be deposited, removed, replaced, and so on. This ability to readily have a supply of phosphorus is vital, considering the huge number of chemical processes in which this element takes a crucial part. The ability to have a supply of calcium equally available is also necessary for many other chemical reactions and the correct balance of the body's fluids. Animals living in salt or freshwater, or a mixture of the two, or moving from one to the other, must be able to cope with the changing osmotic situation. Adjusting the calcium level in bodily fluids is essential to keeping the internal environment stable, whatever is happening to the external environment. The bone acts helpfully both as a storehouse and as a dumping ground. At different times the two aspects have equal merits. Still, the ability to control these vital elements, both so crucial to metabolism, has created the argument that bone's metabolic functions are primary to its structure and support functions. Admittedly, each is vital, and pride of place is entirely hypothetical. Still, the argument points out that bone is not the static framework of structure and support that a skeleton might seem to be. It is not comparable with the permanent frame of an office block.
Moreover, unlike most structures, the skeleton has to provide firm rigidity and extreme flexibility. The muscles must be strongly anchored, yet there must be articulation. As with so much else dictated by all the opposing forces of natural selection, the result is compromised. Man is a mixture of stiffness and relaxation, restriction and freedom, joints without movement and joints with great versatility. The wrist is free, the ankle far less so. The thumb is opposable, but the big toe is not. Just as an architect indicates on his drawings how a door will open, so have attempts been made to indicate all possible human movement. The result is always chaos. The hand can touch everywhere else on the body, even most people, but it is hard to show this. It is harder still to show how a child's ability to squat, with an apparent total disregard for any knee or ankle joint limitations, gradually loses this rubbery simplicity and is transformed into the more awkward posture of the typical adult. A child's elasticity is particularly important at birth when even its cranial bones can be moulded, and the narrow passage of the birth canal is not as restrictive as would otherwise be the case.
Considering that the hand of man has played an integral part in man's development and that both its structure and nervous connections are more highly advanced than in any other creature, it still has great limitations. Nothing can be done with any of its fingers except flexion and extension. The thumb has slightly greater freedom, and the customary explanation of the human hand's dexterity is its capacity to oppose itself to every finger. With a little cooperation from each finger, the thumb tip and fingertip can be made to touch. However, the fact that the hand possesses two distinct grips is believed to be even more remarkable. Two billiard balls, for example, can be picked up, and each is held independently by two parts of the same hand. One is held between the last two fingers and the palm; the other is gripped between the thumb and forefinger. Being able to hold two balls in this manner may seem a poor asset, but it is certainly not unimportant that these two grips exist. Try taking the top off a pen with the hand holding it, and imagine doing it without two grips. Try doing anything with only the single, claw-like grip of a baby's hand, and imagine such restriction.
Incidentally, about the hand, the professions have been confused by nomenclature for the five digits. Elsewhere in the body, the anatomists never seem to have been at a loss, but the medical and legal professions, plus the ordinary citizen, do not agree about nomenclature. Courts of law, insurance documents, doctors and mere people should at least be referring to the same digit, but such unanimity is frequently lacking. Is the thumb the first finger, or is the forefinger? Do we have four fingers or five? Is the order thumb, index, middle, ring and little? If so, where is the fore-finger, and where is the first finger, and do all people wear rings on the left or right hand? And how many people in
Does England know that the pinky is an American little finger? By the end of 1965, every one of the fifty States of the Union except for California had adopted standard systems of naming the five (or four) fingers. Still, in their collective attempts at standardization, they used ten different systems. The Lancet said every system had its drawbacks and wondered about a man `who has lost one www.angered polydactyl but having no properly differentiated thumb'. The dictionaries, in attempting to clarify, outline the problem. In general, they say a finger is 'one of the five terminal members of the hand or one of the four other than the thumb'.
Finally, some of the earliest human-type skeletons indicate human-type behaviour. According to Bones, Bodies and Disease (by Richard Fiennes), skulls of Australopithecines have been found showing a form of fracture consisting of two depressions close together. The humerus bone of antelopes has been discovered nearby. The end of such a bone fits well into the skull depressions. In other words, a million years ago, people were already being hit on the head by people. More recent burial grounds and skeletal remains give permanent evidence of humankind's perpetuation of this custom.