COMPOSITION OF BONES. 107 
jose division, the investigations of the student would be 
ereatly facilitated by its construction, as the character 1s ob- 
vious and defined. This, unfortunately, is far from bemg 
the case. The scales on the body of the armadilio, 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. VII. 
ON THE OSSEOUS SYSTEM, INCLUDING THE COMPOSITION, 
ARTICULATIONS AND ARRANGEMENT OF BONES. 
Tue organs which we examined in the preceding chapter, 
are generally considered as destined for protection, and are, 
therefore, placed on the exterior of the bedy. The bones 
occupy a different position. They are scated 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, cartilagmous basis, earthy matter, and fat. 
