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 np of the tarsus, or ankle ; 

 the metatarsus, or loM^er 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. 



