XIII. — OSTEOLOGY OR SKELETONS. 



107 



The Foot has five bones as in the hand, but they are curved to 

 form the instep. The Toes have bones similar to the fingers in 

 number, but modified in shape to suit the different uses to which 

 they are put. The foot is supported on three points, the heel, the 

 ball of the great toe, and the ball of the little toe. The arch is 

 sustained by a strong ligament which stretches from the heel to 

 the toes. 



The Skeleton in the living animal is covered with about 400 

 muscles, which are attached by sinews to the bones. Motion is 

 obtained by the contracting and thickening of these muscles and 

 is imparted to the bones through the sinews. The ends of the 

 bones at the joints are covered with a fine smooth layer of gristle, 

 and they are attached to each other by strong ligaments. Most 

 of the bones are hollow, and many contain a fatty substance 

 called marrow ; they are covered in life with a thin skin well sup- 

 plied with blood vessels, and the joints are kept soft by a fluid 

 resembling oil. 



Such is the human Skeleton. In the lower vertebrata various 

 modifications are found. In Quadrupeds the arms become legs, in 

 Birds they become wings, in Fishes both arms and legs are represented 

 by fins ; in some animals the vertebrae are greatly increased in 

 number by the addition of a tail, and there are many other dif- 

 ferences which an inspection of the specimens in the Cases will 

 show. The following short table has been prepared to assist in com- 

 paring the number of vertebrae in some of the higher animals : — 



* Sometimes 21. t And often considerably over that number. JHas no sacral vertebrae. 



