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 liave 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 
vertebree is enormous, the number of vertebrz increases pro- 
digiously ; while in the frog, where progression is so carried on 
as scarcely to affect the spinal column, the vertebre are sur- 
prisingly few. Among birds, too, where the number of vertebree 
is extremely variable, it is found that those genera which use 
their cervical or sacral regions most, have in those regions most 
vertebrae: thus the emu and cassowary have each nineteen sacral 
vertebra, while the emu has as many in the neck. And while 
the swan has twenty-three cervical vertebra, 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 vertebrze 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 vertebre. 
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 vertebre 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 Plestosaurus, whether called 
pleurapophysis or heemapophysis, is extremely short in the neck, 
