STRUCTURE OF THE HUMAN BODY. 47 



Earthy portion of bone. 



20. {The proportion between the animal and earthy sub- 

 stances varies in different individuals, and in the same indi- 

 vidual at different periods of life, and under various condi- 

 tions of health} In youth, the former, in old age, the latter 

 predominates. The earthy matter is sometimes so deficient 

 that jthe bones have not the necessary degree of firmness 

 and rigidity. |*The consequence is, that those parts of the 

 skeleton which have to support any considerable weight 

 bend under it, as the spine, the bones of the pelvis, and the 

 lower limbsy This is one cause of spinal distortions, though 

 they are generally caused by debility of the muscles from 

 want of proper exercise. In many cases of the former kind, 

 by the subsequent deposition of earthy matter, the bones be- 

 come sufficiently dense and compact ; but the distortion 

 remains fixed and permanent, and mechanical means, such 

 as pulleys, screws, inclined planes, &c., only serve to torture 

 the patient, without affording the slightest prospect of a 

 cure. No one who understands physiology will attempt to 

 correct such distortions by mechanical means. |Where 

 there is a deficiency of animal matter, the bones want a 

 poper degree of tenacity, and are therefore easily fractured 

 by slight blows or fallsj 



21. The ends of the bones which are used for motion, as 

 already mentioned, are tipped with cartilage or gristle, which 

 is very smooth and hard ; and it is constantly wet, in a state 

 of health, by a fluid which answers the same purpose as oil 

 in machinery, or tar upon wagon wheels. The joints are 



