THE SKIN AND SKELETON. 



147 



shin-bone ; and the fibula, or splint-bone. The rounded 

 head of the femur fits into a cavity (acetabulum} in the 

 pelvic arch, while the lower end articulates with the tibia, 

 and sometimes (as in Birds) with the fibula also. An ex- 

 tra bone, the patella, or knee-pan, is hung in a tendon in 

 front of the joint between the femur and tibia of the high- 

 er animals. The foot is made up of the tarsus, or ankle ; 

 the metatarsus, or lower instep ; and the phalanges, or 

 toes. The toes number from 1 in the Horse to 5 in Man. 

 Certain parts of the skeleton, as of the skull, are firmly 

 joined together by zigzag edges or by overlapping; in 

 either case the joint is called a suture. But the great 

 majority of the bones are intended to move one upon an- 

 other. The vertebrae are locked together by their proc- 

 esses, and also by a tough fibrous substance between the 

 centra, so that a slight motion only is allowed. The limbs 

 furnish the best examples of movable articulations-, as the 

 ball-and-socket joint at the shoulder, and the hinge-joint 

 at the elbow. The bones are held together by ligaments,- 

 and, to prevent friction, the extremities are covered with 

 cartilage, which is constantly lubricated with an unctuous 

 fluid called synovia. 



CHEMICAL COMPOSITION OF BONES. 



