16 THE MECHANISM OF LIFE 



period. But with the increase in mass of the body there occur 

 certain mechanical disabilities, and so we must regard the huge 

 reptiles of past geological times as some of Nature's unsuccessful 

 experiments. 



Now we may turn to the skeleton of the present-day verte- 

 brate animals, and here we find the most perfect mechanism so 

 far evolved. 



The Axial Skeleton. — The whole structure is built up round 

 an axis, which is the vertebral column, or backbone. This 

 consists of a series of vertebrae, or bony discs, which are attached 

 to each other by means of muscles and ligaments, and they are 

 usually kept in place by little bony projections that articulate 

 with each other. In man there are pads of gristle between the 

 adjacent vertebrae, and the latter have a certain amount of play 

 on each other, so that the whole column can bend sideways and 

 from back to front. In some fishes, however, the latter is prac- 

 tically one bone; in birds only the vertebrae of the neck are 

 movable on each other ; and in snakes the separate bones can 

 move only in the horizontal plane, because of little interlocking 

 side pieces. In front the vertebral column carries the skull, and 

 behind it is often prolonged backwards to form the tail. 



The skull consists of two series of parts, the bones of the 

 cranium and those of the face and jaws. The cranium is essen- 

 tially a bony box containing the brain and the great sense organs, 

 eyes, auditory and olfactory organs. Added to it are the bones 

 of the upper jaw (which are immovable on the skull in man) and 

 the lower jaw, which articulates with the skull by means of a 

 hinge- joint (but, nevertheless, allows of a certain amount of side- 

 to- side movement). The whole skull is attached to the first 

 (atlas) vertebra by a hinge- joint, so that it can move up and 

 down in a vertical plane, and the atlas is attached to the second 

 (axis) vertebra by a peg- and- socket joint, so that the skull can 

 be rotated through a certain angle in the horizontal plane. The 

 neck vertebrae can bend on each other from side to side, and thus 

 the skull has limited freedom of movement in the three directions 

 of space. 



The ribs are imperfect hoops of bone which are articulated to 

 the backbone behind. Some of them (the upper ones) are also 

 articulated in front to the breast-bone, and thus the whole series 

 act as an outer protection for the organs of the thoracic cavity — 



