The skeleton. 21 



and fat or marrow, and the latter of phosphate of lime In con- 

 siderable proportion, a lesser quantity of carbonate of lime, and 

 a small portion of other salts. The cartilage of bones is formed 

 before the earthy matter, and constitutes in fact the nidus in 

 which the latter is deposited. Bones can be freed from their 

 earthy portion by immersion in an acid, by which process the 

 gelatine is also dissolved, and pure cartilage is left, which is 

 elastic, but retains the original figure of the bone. On the 

 other hand, bones by exposure to a great heat are deprived of 

 the animal substances, and the earthy part remains. 



The use of the marrow is more particularly to i)revent the too 

 great dryness and brittleness of bones. To the animal portion 

 of their composition are they, therefore, indebted for their shape 

 and what degree of elasticity they possess, and from the earthy 

 portion they derive the important purposes of strength and sta- 

 bility. Thus are these different elements combined together, 

 and by an union of their different principles form a substance 

 admirably adapted for affording full scope for the play of the 

 various organs of life, protecting at the same time the vital parts 

 from external injury, admitting and assisting the powers of 

 locomotion, and, in fine, forming a secure fabric for the beautiful 

 building of animal frames. Every bone Is covered by a mem- 

 brane called the periosteum, which also lines the internal ca- 

 vities, and secretes the marrow : Its use Is to circumscribe the 

 form of bones, and protect them by Its tenseness, as well as to 

 afford the medium whereby they are furnished with their vessels. 

 The shape of particular bones intimately corresponds to the 

 purpose for which they are Intended ; where for the office of pro- 

 tection, we findtliem flat; and where for the purpose of motion, 

 long and cylindrical, as In the extremities. In the construction of 

 tlie skull the most perfect mechanism Is displayed. The first object 

 to be obtained Is the protection of the brain from the accidents 

 to which, from the peculiarities of animals, It is mostly exposed. 

 For this purpose the skull consists of two tables or ])lates; the 

 outer thick and tough, the Inner hard and brittle : the former, by 

 yielding in a measure to resistance, diminishes concussion, whilst 

 the latter by its hardness prevents sharp bodies from penetrating 

 to the brain. Now, If these two plates were reversed, the brittle 

 would not only be in great danger of fracture, but would also 

 vibrate considerably ; and the injurious effect of this vibration 

 may well be conceived when we find that, even with the pre- 

 sent wise precaution, it often occasions greater mischief than the 

 most serious fractures. Between the skull and the brain are 

 interposed several membranes, which also assist materially in 

 preventing vibration, just as a piece of parchment would inside a 

 rummer glass. The skull is composed in animals of upwards of 

 thirty bones, which are connected together by a dovetailed 



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