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 
