356 COMPARATIVE ZOOLOGY 



distinct in amphibians, reptiles, and in the young of 

 higher animals ; but in adult birds and mammals they 

 become united together, and are also (except in whales) 

 solidly attached to the sacrum. The two pelvic arches 

 and the sacrum thus soldered into one make the pelvis. 

 The leg bones consists of \hzfemur, or thigh ; the tibia, 

 or 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 extra bone, the patella, or kneepan, is hung 

 in a tendon in front of the joint between the femur and 

 tibia of the higher animals. The foot is made up of the 

 tarsus, or ankle ; the metatarsus, or lower instep ; and 

 faz phalanges > or toes. The toes number from i 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 

 another. The vertebrae are locked together by their 

 processes, 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. 



A chemical analysis of bone shows it to consist mainly 

 of phosphate and carbonate of lime and phosphate of 

 magnesia mingled with glutin, chondrin, and oil, the 

 amount of each varying in different animals. 



