THE SKIN AND SKELETON 347 



ties (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 most birds 

 and in skulls contain air. 



The number of bones not only differs in different 

 animals, but varies with the age of an individual. In 

 very early life there are no bones at all ; and ossifica- 

 tion, 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 centers of ossification. 

 But the actual number is usually much less a result 

 of the tendency of these centers to coalesce. Thus, the 

 thigh bone in youth is composed of five distinct por- 

 tions, which gradually unite. So in the lower verte- 

 brates many parts remain distinct which in the higher 

 are joined into one. The occiput or bone at the base 

 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 backbone ; but in the development of 

 the various parts, and especially of the appendages, 

 there is endless variety. Fishes possess a great number 

 of skull bones, but have no fingers and toes. The snake 

 has plenty of ribs and tail, but no breastbone ; the frog 

 has a breastbone, but neither tail nor ribs. As the skele- 

 ton of a fish is too complicated for the primary student, 

 we will select for illustration the skeleton of a lion 

 the type of quadrupeds. It should be remembered, 

 however, that all vertebrates are formed on one plan. 



In the lowest vertebrate, amphioxus, the only skele- 

 ton is a cartilaginous rod running from head to tail. 



