224 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



consists of cartilage bone, but the kneecap and several 

 small bones in the joints are tendon bones, while the 

 teeth and scutes are examples of skin bones. Bones 

 are covered with a strong membrane called the peri- 

 osteum. 



Number of Bones in Man. — The number of bones 

 in the human skeleton is variously estimated from a little 

 over two hundred to about two hundred and fifty, the dif- 

 ference being the result of difference of view as to what 

 ought to be included. Some include the. teeth, and some 

 do not, because these are gum structures. Some include 

 the sesamoid or joint bones, some do not. Again many 

 pieces are separate in the embryo, and become consoli- 

 dated later. These two hundred or more pieces are ar- 

 ticulated together into a complex structure, which is 

 moved by the muscles. 



Joints. — The articulations or joints are of two gen- 

 eral kinds, fixed and movable. In fixed joints the pieces 

 may be cemented together with cartilage (symphyses), 

 as the hip bones with the sacrum, or they may be inter- 

 digitated or dovetailed (suture), as in the bones of the 

 skull. The movable joints are also of two kinds — the 

 hinge joint, when motion in one plane is required, as in 

 the knee, elbow, etc., and the ball-and-socket joint, where 

 universal motion is required, as in the shoulder and hip. 

 Only the movable joints concern us here. 



Movable Joints.— The bones in a movable joint 

 are (a) enlarged at the ends where they come together, 

 so as to make a firmer contact, (b) They are covered 

 with cartilage for perfect smoothness and elasticity, (c) 

 The joint is inclosed in a capsule of strong fibrous tis- 

 sue to prevent displacement, and at the same time ex- 

 clude the air. (d) The closed cavity thus formed is 

 lined with a serous membrane, which secretes a smooth, 

 glairy, lubricating fluid — the synovia or joint juice. 



