136 COMPARATIVE ZOOLOGY: 
‘tissue. Yet there is a diversity in the composition of 
bony skeletons: that of fresh-water Fishes contains the 
least earthy matter, and that of Birds the most. Hence 
the density and ivory-whiteness of the bones of the lat- 
ter. 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 attachment 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 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 three or four bones, 
which are seen separate in the skull of the Fish. 
A complete skeleton, made up of all the pieces which 
might enter into its composition, does not exist. Every 
animal has some deficiency. All have a skull and back- 
