138 COMPARATIVE ZOOLOGY. 



whiteness of the bones of the latter. Unlike the shells of 

 Mollusks and the crust of the Lobster, which grow by the 

 addition of layers to their borders, bones are moist, living 

 parts, penetrated by blood-vessels and nerves, and covered 

 with a tough membrane, called periosteum, for the attach- 

 ment of muscles. 



The surface of bones is compact ; but the interior may 

 be solid or spongy (as the bones of Fishes, Turtles, Sloths, 

 and Whales), or hollow (as the long bones of Birds and 

 the active quadrupeds). There are also cavities (called 

 "sinuses") between the inner and outer walls of the skull, 

 as is remarkably shown by the Elephant. The cavities in 

 the long bones of quadrupeds are filled with marrow ; 

 those in the long bones of Birds and in skulls contain air. 



The number of bones not only differs in different ani- 

 mals, but varies with the age of an individual. In very 

 early life there are no bones at all; and ossification, or 

 the conversion of cartilage into bone, is not completed 

 until maturity. This process begins at a multitude of 

 points, and theoretically there are as many bones in a 

 skeleton as centres of ossification. But the actual number 

 is usually much less a result of the tendency of these 

 centres to coalesce. Thus, the thigh-bone in youth is 

 composed of five distinct portions, which gradually unite. 

 So in the lower Vertebrates many parts remain distinct 

 which in the higher are joined into one. The occiput or 

 back-bone of Man's skull is the union of four bones, which 

 are seen separate in the skull of the Fish, or of a baby. 



A complete skeleton, made up of all the pieces which 

 might enter into its composition, does not exist. Every 

 Vertebrate has some deficiency. All, except Amphioxus, 

 have a skull and back-bone; but in the development of 

 the various parts, and especially of the appendages, there 

 is endless variety. Fishes possess a great number of skull- 

 bone.% but have no fingers and toes. The Snake has plenty 



