0)1 the Origin of the Vertebrate Skeleton. 21 



III. — The Origin of the Vertebrate Skeleton. 

 By Haery G. Seeley, St. John's College, Cambridge. 



[Continued from vol. ix. p. 280.] 



§ 3. The Physics of the Skeleton. 



The next step after a study of growth is to observe in what 

 directions growth usually occurs ; then we may discover the 

 forces which accumulate the energy that results in such growth. 

 All animals of the kinds named Vertebrata have their internal 

 bones arranged in a way which in many respects is the same 

 for them all — a great antero-posterior extension ; and this 

 arrangement is named the skeleton. But when animals are 

 contrasted with each other, they manifest differences in the 

 degree of growth, and in the presence or absence of some of 

 their bones ; and these peculiarities, being persistent through 

 an immense number of variously modified individuals, give to 

 the skeleton a number of different plans, which admit of being 

 defined. And out of these considerations arise the great pro- 

 blems affecting all bones, which will here be stated. They 

 are : — What is the skeleton, and why has it an existence as a 

 skeleton ? and what are the plans of growth of the skeleton 

 among vertebrate animals, and why do those plans exist ? 



Here, then, the skeleton first appears as an accomplished 

 fact, without visible genesis beyond such as may be traced in 

 each individual, where changes are observed to occur in the 

 bones after an animal has left the &gg or the uterus, which 

 are in sequence from their first formation to completed growth. 



By the skeleton, I understand in the foregoing passage the 

 vertebrate skeleton only ; and I wish, for convenience, to keep 

 the idea of the vertebrate skeleton distinct from other impor- 

 tant osseous machinery of vertebrates, which is better named 

 the appendicular bones, the dermal bones, and the respiratory 

 bones. The reason for this distinction is that the nature of 

 their relation to the axial skeleton must first be demonstrated 

 before it can be reasoned upon. The vertebrate skeleton, 

 moreover, is the only one which is well developed in every 

 vertebrate animal, the other bones being variable and giving 

 characters to the plans of the subordinate sections. Thus the 

 Vertebrata admit of being defined as those animals in which 

 the elongated central nervous system is sheathed posteriorly 

 by a sequence of osseous rings, and anteriorly by a bony box — 

 the rings being the vertebrai protecting the spinal cord, while 

 the box is the skull covering the brain. This definition in- 

 cludes all the animals classed by zoologists as Vertebrata, ex- 

 cepting the lancelet {AmpMoxus)^ which, for reasons givxn in 



