22 



CHAPTER III. 



STRUCTURES OF THE BODY. 



Bones — Cartilage — Muscles and Tendons — Ligaments — Connective Tissue — -The 

 Nervous System. 



The frame-work of the body consists of the skeleton (Fig. 

 i8), which is composed of a large number of connected 

 bones that are moved by muscles. We should, generally, 

 regard differences of length in the limbs, as those of muscles, 

 rather than of bones ; for the former are the producers 

 of all movements of progression, the latter aiding only 

 in the application of the force exerted. 



Bones. — The chief duties of hones in the act of pro- 

 gression are — (i) to bear weight ; (2) to resist the effects 

 of concussion ; and (3) to act as levers. Capability for 

 performing (i) and (2) is dependent on conditions of 

 texture (" quality "), size (" substance "), and arrange- 

 ment. 



Composition, qUality and substance of bone. — Bone is 

 composed of about one-third of animal matter, and two- 

 thirds of earthy matter, more than 98 per cent, of which 

 consists of salts of lime, and principally of phosphate of 

 lime. ''The animal and earthy constituents of bone are 

 so intimately blended and incorporated the one with the 

 other that it is only by severe measures, as for instance 

 by a white heat in one case and by the action of con- 

 centrated acids in another, that they can be separated. 

 Their close union is further shown by the fact that when 



