204 CONSTRUCTION OF THE JOINTS 



little weight, for they are spongy in their texture. The 

 flat bones are used to form the walls of cavities, as the 

 skull. The long bones are found in the limbs, and 

 are entirely employed in locomotion. 



Emily. How are bones connected together at the 

 joints, so as to give them the wonderful strength, as well 

 as facility of motion which they enjoy ? If I am not 

 deceived, I suspect there is much that is exceedingly 

 curious in the construction of these parts. 



Dr. B. Yes the structure and use of the joints do 

 present us some very interesting facts. They are distin- 

 guished into those that are moveable, and those that are 

 immoveable. A specimen of the latter, we have in the 

 teeth and the head ; but it is the former kind that we 

 wish more particularly to examine. Physiologists have 

 pointed out two kinds of the moveable joint ; the ball and 

 socket joint, and the hinge. Of the former we have a 

 specimen in the hip-joint. The head of the bone pre- 

 sents a large round surface, which is received into a deep 

 cup-like socket in the hip-bone, in which it plays with 

 an easy motion in all directions. In the elbow is an 

 instance of the hinge-joint. The fore-arm bends for- 

 ward on the arm, but in its motion backward, it is stop- 

 ped when on a line with the arm, by a process of bone 

 which juts out from its extremity and comes in contact 

 with the other bone. 



In the construction of the joints, every thing shows 

 the regard that has been paid to the security and easy 

 motion of the parts thus connected together. The ends 

 of the bones are, in the first place, surrounded by a thin 

 coating of firm ahd elastic cartilage. 



Emily. I do not see how this can produce an easier 

 motion, than if the ends of the bones were naked and 

 perfectly smooth. Besides, cartilage being a substance 

 less hard than bone, would certainly wear out sooner. 



Dr. B. But though sooner exhausted, it is at the 

 same time, more speedily repaired, than bone. Provi- 

 sion for preventing the effects of friction, can be more 

 easily provided also, in case of cartilages than bones. 



