Human Bone

Origin of The Skull
Many invertebrate animals (for example, earthworms and caterpillars) have bodies divided into a large number of similar segments. Vertebrate animals are also segmented. This is most clearly seen in fish, but is partly obscured in mammals such as ourselves. The head of a fish is not obviously segmented, but the swimming muscles are divided into w-shaped blocks called myomeres, which separate easily when the fish is cooked. There is a vertebra for every pair of these blocks (one block on each side of the body), and a pair of nerves emerges between each vertebra and the next. Thus the body of the fish, behind the head, is a series of segments, each with a pair of myomeres, a vertebra and a pair of spinal nerves. The segmental structure of the muscles of the torso is less obvious in mammals such as ourselves, and we have more variation in structure between successive vertebrae, but the basic segmental structure remains. Human embryos have myomeres, just like the myomeres of fish embryos.

It has been suggested that segments that were formerly part of the trunk may have been incorporated in the heads of vertebrates. In 1791, Johann Goethe, the great German poet, picked up a sheep’s skull in the Jewish cemetery in Venice. He was an anatomist as well as a poet, and it occurred to him that the skull was a series of modified vertebrae. He did not publish the idea until 1820, by which time the German nature philosopher Lorenz Oken had independently formulated and published the same idea. Their theory did not imply evolution (Charles Darwin’s The Origin of Species did not appear until 1859), merely that the skull was in some fundamental way equivalent to a series of vertebrae. It was discredited by the English biologist Thomas Huxley, who was to become Darwin’s most vocal supporter, in 1858. That theory was abandoned, but a succession of later anatomists formulated a less extreme theory. This new theory was presented in its most developed form in 1930 in Edwin S. Goodrich’s Studies on the Structure and Development of Vertebrates. This theory will bring us back to cranial nerves (the brain sends out many nerves, known as cranial nerves, through holes in the skull).

Palate and Jaws
The palate is a plate of bone that forms the floor of the nasal cavity and the roof of the mouth cavity.  It protects the delicate turbinals from damage by food in the mouth, and it forces the air to pass over all the turbinals before it can reach the mouth cavity.  The opening of the nasal cavity into the mouth is well back in the mouth, close to the opening of the windpipe at the top of the throat.  Thus the air by-pass the mouth cavity, making it possible for us to breathe with our mouths full.  This is very helpful, when food needs prolonged chewing.  Chewing food when your nose is blocked by a heavy cold can be a most uncomfortable experience.  Other mammals have palates like ours, but the nostrils of most reptiles and of birds open directly into the front of the mouth.  They do not need the bypass arrangement because they do not chew their food.

The upper jaw and palate develop before birth, from bones that grow inward from either side of the head.  Occasionally the two sides fail to meet, causing the defect known as cleft lip and palate.  This can be corrected by surgery.

Tooth Structure
We have 32 teeth, of four different kinds.  At the front are the square, sharped-edged incisors, two on each side in each jaw.  They are good for biting off pieces of food.  Next are the pointed canines, one on each side in each jaw.  In carnivores, the canines are the fangs that are used to kill prey and tear flesh, but our canines are much too short for those purposes.  Their pointed shape is a relic of our ancestry.  The canines are followed by two premolar teeth on each side in each jaw.  Whereas the canines have a single pointed cusp, the premolars each have two cusps side by side.  Finally, behind the premolars, are the molars.  There are three of them on each side of each jaw.  The molars have four cusps, placed as the corners of a square.  We use our premolars and molars for chewing.

All the teeth have roots, which are firmly fixed by collagen fibres in their sockets in the jaws.  The incisors and canines have one root each; most of the premolars have one but the first upper ones have two; and the molars have two or three roots.  All the teeth are hollow, with a pulp cavity filled with soft tissue.  Blood vessels and nerves enter the pulp cavity through openings at the tips of the roots.

When the mouth is closed in its normal resting position, the crowns of the upper and lower molars are in contact, but the lower incisors lie behind the upper ones.  The upper and lower molars fit neatly together, with the cusps of the upper molars in the hollows of the lower ones, and vice versa.

Teeth consist mainly of dentine (ivory), which is very similar in composition to bone.  The main difference is that bone has living cells scattered all through it, but dentine has all its cells in the soft tissue of the pulp cavity.  The crowns of the teeth are covered by a layer of enamel, 2.5mm thick in its thickest parts.  Enamel is harder than bone and more resistant to wear, but also more brittle.  Once they have erupted, our adult teeth have to last for the rest of our lives, so wear resistance is very important.  They also survive well after death, better than any other part of the body.  Teeth are the commonest fossils of humans and other vertebrates.

Engineering Vs Evolution
The Design of the skeleton reflects ancestry as well as function.
The functions of different parts of the skull can be traced back to our ancestry. For example, our canine teeth are the rudiments of our ancestors’ fangs. The cranial nerves, which emerge through holes in the skull, reflect the segmented structure of the earliest vertebrates. The pattern of bones in the roofs of our skulls has not been designed afresh by evolution, to suit the particular needs of the human race, but has changed only a little from the pattern in our amphibian ancestors of 350million years ago.

There is a fundamental difference between the processes of evolution and of engineering design. An engineer who has designed a boat, and wants next to design a car, goes back to the drawing board, and builds up the new design from scratch. Evolution, however, starts from an existing design and alters it progressively by a series of small changes over many generations. The final product(eg. amphibian) may be very different from its ancestor (eg. fish), but every stage in the evolutionary sequence must be an effective design, capable of holding its own in a competitive world. As a result, many of the features of a species may not be ideal for its way of life, but may be (more or less) the best that evolution could do, starting from that species’ ancestors.

Evolution by natural selection (the survival of the fittest) may be incapable of taking an animal from a subsidiary peak of fitness to a higher peak, because that would require a descent into a less fit state. The pattern of bones in human skull roofs is not necessarily the best possible for us, but is a pattern that could be evolved from our ancestors.

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