112 The Vertebrate Organ Systems 



Structures with a similar origin, but often with dissimilar func- 

 tions, are termed homologous. This principle of homology is one of 

 the basic biological concepts that has enabled scientists to interpret 

 many otherwise obscure relationships. One of the clearest examples 

 of this can be seen in a study of the forelimbs of the various verte- 

 brates. The wing of the bird, the flipper of the whale, the arm of 

 the human being, and the digging hand of the mole superficially 

 appear quite different, but basically have the same bones in the same 

 relationship to one another. 



Fig. 33. — Diagrams illustrating homology and analogy. A, Arm of man is ho- 

 mologous with wing of bird, B, while B is analogous with C, the wing of an insect. 



By contrast there are certain structures with quite dissimilar origin, 

 but with similar functions. These are termed analogous structures. 

 Some analogous structures are the gills of the fish and the lung of 

 the mammal; the wing of the bird and that of the insect. Homology 

 applies to structure, and analogy applies to function. Care must be 

 taken not to confuse these two terms. 



4 



4 



JOINTS 



The region at which two bones meet is known as a joint. There 

 are two recognizable groups of joints. The first of these two are 

 the synarthroses in which there is little or no movement of the adjoin- 

 ing bones. The union of the bones of the skull and those of the 

 pelvic girdle are examples of these. The point of junction between 



I 



I 



