352 Mr. H. G. Seeley on a Theory 



as it would be if the tarso-metatarsal explanation were accepted ; 

 for, terminating in the Achilles tendon, it is eminently the 

 muscle of the os calcis. And, seeing how the os calcis is elon- 

 gated by it in ordinary mammals, one cannot be blind to the 

 fact that, if the tension were increased to a power many times 

 as great as it is in mammals, the bone would be extended to 

 a much greater length. And therefore, when there is such 

 a great power as this huge muscle present in birds, capable of 

 elongating the tarsal bones, I fail to see any reason for supposing 

 that the laws of osteological development have been departed 

 from in birds. Therefore, when the muscles become of sufficient 

 power, there is every reason to believe that the tarsal bones will 

 follow the same law as other bones, and become elongated, de- 

 veloping a shaft ; and hence, and for reasons indicated, under 

 ordinary circumstances they present the condition of epiphyses 

 of bones where the shafts are never formed. 



And all these considerations point alike to the same general 

 conclusion, that one ossification may develope another, if suffi- 

 cient pressure and tension can be applied to its surface. And 

 this law appears to be equally true for the entire animal as for 

 a single bone. Thus in serpents, where the tension on the 

 vertebrae is enormous, the number of vertebrse increases pro- 

 digiously; while in the frog, where progression is so carried on 

 as scarcely to affect the spinal column, the vertebrse are sur- 

 prisingly few. Among birds, too, where the number of vertebrse 

 is extremely variable, it is found that those genera which use 

 their cervical or sacral regions most, have in those regions most 

 vertebrse : thus the emu and cassowary have each nineteen sacral 

 vertebrse, while the emu has as many in the neck. And while 

 the swan has twenty-three cervical vertebrse, and the average of 

 this region in Natatores, Grallatores, and Cursores is much 

 higher than in the other orders, on the other hand, in birds of 

 great flight the number of vertebrse is small. Such facts appear 

 to lead to the conclusion that the different regions of the body 

 most used experience in consequence a tendency to increase in 

 development. 



With these remarks on the relation of structures to functions 

 we may now examine the constitution of the vertebrse. 



The body of the vertebra, or centrum, follows the law of a typi- 

 cal bone, and is therefore made up of two epiphyses and a shaft. 

 And when it is seen with what ligaments the vertebrse are con- 

 nected, to what vibrations they are subject in motion, and what 

 muscles bind them together and pull them about, these powers 

 are the forces which develope and account for the epiphyses. 



The rib in a typical animal, as a Plesiosawus, whether called 

 pleurapophysis or hsemapophysis, is extremely short in the neck, 



