of the Vertebrate Skeleton. 25 



tension, and to examine how the part of the body under con- 

 sideration can be affected by these forces. 



It is due to Mr. Herbert Spencer to state that he has 

 endeavoured to grapple with this question ; but, although he 

 appreciated fully the simple mechanical conditions of the 

 problem, he seems to me to have failed to solve it. His argu- 

 ment is that when pressure is manifested on alternate sides of 

 a rod, there will be a neutral axis within it which only expe- 

 riences small compressions, and external to that an investing 

 region, where pressure and tension alternate. He then tries 

 to apply that principle to a fish. The principle would be 

 perfectly applicable to a long bone, and would account for its 

 being hollow or less dense internally ; but it is not applicable 

 to a fish, because there is nothing to correspond to the hollow- 

 ness of a bone in the middle line of the animal ; and, on the 

 contrary, the region which should be the unossified neutral 

 axis is the ossified neural skeleton — a condition exactly the 

 reverse of what it should be were Mr. Spencer's hypothesis 

 true. Mr. Spencer's error consists in not recognizing that the 

 muscles of the body are, in regard to the production of the 

 neural skeleton, precisely what the weight is which bends a 

 revolving flexible rod — the power which produces a neutral 

 axis, and which also produces the pressure and tension in 

 which we have seen that ossifications arise*. 



In seeking to explain this formation of an osseous skele- 

 ton, instead of taking an abstract, impossible archetype to 

 reason from, my argument may be clearer if we examine 

 the conditions of the problem as presented in some animal. 

 Having the choice of animals, among which a Chelonian 

 would be the least suitable, the most difficidt skeleton to un- 

 derstand, I select a whiting. The fish manifests locomotive 

 energy 5 and to find the source of this mechanical power, I 

 skin her. The skin requires to be dissected off, on account of 

 its close union with the constituent fibres of the muscles ; and 

 in some parts of the body there are attached to it special 

 skin-muscles in addition. The skin removed, there is seen an 

 enormous development of muscles, which are arranged in a 

 very marked way. Fibres extend from the skin obliquely 

 inward toward the skeleton ; and these fibres are grouped into 

 obliquely placed muscles, which are arranged along the animal 

 parallel to each other, so as to make large strips of similar 

 muscles, which reach from tail to head. In the tail of the 

 whiting there are four of these strips on each side ; and tlie 

 constituent muscles are so arranged that the obliquity of the 



* Principles of Biology, vol. ii. p. 196. 



