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Some animals are entirely destitute of hard parts: this is the case with the 

 zoophytes, some worms and insects. The internal structure of bones is 

 composed of nearly the same materials in all animals: viz. gelatine and 

 salts containing- a calcareous basis. The external skeleton of white-blood- 

 ed animals bears a much greater resemblance to the epidermis than to 

 the osseous system of the red-blooded animals. Like the epidermis, it 

 undergoes changes of decomposition and renovation. Thus, the lobster 

 parts with its shell, every year, when the body of this crustaceous animal 

 increases in size, and it is replaced by a new envelope, which is, at first, 

 very soft, and which gradually acquires the same consistence as the 

 former. Lastly, the skeleton of birds differs from that of all other ani- 

 mals, in having its principal bones pierced by openings communic'iting 

 with i he lungs, and always filled with an air rarified by the vital heat, 

 winch greatly assists in giving to them that specific lightness so essential 

 to their peculiar mode of existence. 



The osseous system serves as a foundation to the animal machine, 

 yields a firm support to all its parts, determines the size of the body, its 

 proportion, its form and attitude. Without the bones, the body would 

 have no permanent form, and could not easily move from one place to 

 another. When, from the loss of the calcareous earth to which they owe 

 their hardness, these organs become soft, the limbs deformed, standing, 

 and the different motions of progression, become after a time impossible. 

 Such are the effects of rachitis, a disease of which the nature is well 

 understood, though we are not the better informed with regard to the 

 manner in which its causes operate, or the medicines which it requires. 



The vertebral column forms the truly essential aud fundamental part 

 of the skeleton; it may be considered as the base of the osseous edifice, 

 as the point in which all their efforts terminate, as the centre on which 

 all the bones rest in their various motions; since every effort or shock, in 

 any way considerable, is felt there. Moreover, it contains in the canal 

 with which it is perforated, the cerebral prolongation, which furnishes 

 most of the nerves in the body. 



In order that it may support all the different parts and at the same time, 

 protect the delicate organ which it contains*, and adapt itself to the va- 

 rious attitudes required by the wants of life, it was necessary that the 

 vertebral column should possess, besides great solidity, a sufficient degree 

 of mobility; it possesses both these advantages, and owes the former to 

 the breadth of the surfaces by which its bones are articulated together, 

 to the size, the length, the direction, and the strength of their processes, 

 and to the great number of muscles and ligaments connected with it: it 

 owes its freedom of motion to the great number of bones of which it is 

 formed. Each single vertebra has but a slight degree of motion, but as 

 they all have the power of moving at once, the sum of their individual 

 motion added together, gives as the result a general motion which is 



'* The peculiar manner in which the vertebree grow, is itself accommodated to the 

 delicacy of the spinal marrow; consisting-, for a considerable length of time, of several 

 pieces divided by cartilages, the circumference of the opening in these bones, becomes 

 enlarged, with the enlargement of the spinal marrow, as we grow older. The circum- 

 ference of the foramen of the occipital bone and that of the first vertebra which cor- 

 respond to the thickest part of the spinal marrow, is, on thai, account, formed of four 

 distinct pieces separated by cartilages in the first of' these bones, and of five pieces in the 

 other. ^Author'* J\"otp. 



