COMPOSITION OF BONES. 107 



lose division, the investigations of the student would be 

 greatly facilitated by its construction, as the character is ob- 

 vious and defined. This, unfortunately, is far from being 

 the case. The scales on the body of the armadillo, and 

 on the tail of a rat, are more obviously annulose than the 

 crust of the crab or the body of the aphrodite. 



This external character, therefore, is somewhat deficient 

 in precision, or rather in exclusive properties ; and when 

 employed to designate a primary division, may mislead or 

 bewilder the student. This is the more to be regretted, as 

 the internal or essential character of the division requires 

 dissection, and is difficult to detect. It is taken from the 

 appearances presented by the nervous system. 



CHAP. VIL 



ON THE OSSEOUS SYSTEM, INCLUDING THE COMPOSITION, 

 ARTICULATIONS AND ARRANGEMENT OF BONES. 



THE organs which we examined in the preceding chapter, 

 are generally considered as destined for protection, and are, 

 therefore, placed on the exterior of the body. The bones 

 occupy a different position. They are seated in the inte- 

 rior ; and while they likewise assist in protecting many im- 

 portant organs, they, at the same time, give stability to the 

 frame, support to the muscles, and afford levers for the exe- 

 cution of locomotion. 



I. COMPOSITION OF BONES. 



In considering the nature of bone, it will be of advantage 

 to the reader, to be made acquainted with its composition, 

 in order to understand its mode of growth. The different 

 parts of which bones consist, may be reduced to four ; the 

 periosteum, cartilaginous basis, earthy matter, and fat. 



