54 ANIMAL MECHANICS 



bone, so that each filament is perpendicular to 

 the pressure made upon it. The surface of the 

 articulating cartilage is perfectly smooth, and is 

 lubricated by a fluid called synovia, signifying a 

 mucilage, a viscous or thick liquor. This is vul- 

 garly called joint oil, but it has no property of 

 oil, although it is better calculated than any oil 

 to lubricate the interior of the joint. 



When inflammation comes upon a joint, this 

 fluid is not supplied, and the joint is stiff, and 

 the surfaces creak upon one another like a hinge 

 without oil. A delicate membrane extends from 

 bone to bone, confining this lubricating fluid, and 

 forming the boundary of what is termed the 

 cavity of the joint, although, in fact, there is no 

 unoccupied space. External to this capsule 1 of 

 the joint, there are strong ligaments going from 

 point to point of the bones, and so ordered as 

 to bind them together without preventing their 

 proper motions. From this description of a single 

 joint, we can easily conceive what a spring or 

 elasticity is given to the foot, where thirty-six 

 bones are jointed together. 



An elegant author has this very natural remark 

 on the joints : "In considering the joints, there 

 is nothing, perhaps, which ought to move our 

 gratitude more than the reflection, how well they 

 wear. A limb shall swing upon its hinge, or 



1 From capsula, a little case, or box. 



