354 Mr. H. G. Seeley on a Theory 
physes of the body of the centrum, dorsal ribs, though the same 
in origin, assume the appearance of separate bones. And thus to 
alternations of pressure and tension and rest, growth of all kinds 
seems to be due. 
If the upper arches of the vertebral column are now examined, 
they will be found united by a much more elaborate system of 
hgaments than the ribs. There is the posterior common liga- 
ment at the base of the arch, the supraspinous ligament above 
the neural spines, the interspinous ligament, the capsular liga- 
ment, and the ligamenta subflava ; and hence it is not surprising 
to find that the neural arches often come close together and 
underlock each other, and that the neural spines are much more 
expanded in antero-posterior extent than is generally the case 
with the ribs. But the neural arches present no correspondence 
with the ribs in size, remaining small and singularly constant in 
character. Development shows that they grow upon the first 
appearance of the film of the nervous column, which growing 
within and resisted by structures without produces the condi- 
tions under which epiphyses are developed. Hence I conclude 
that the lateral halves of the neural arch are also of the nature 
of epiphyses. But the neural spine, in those animals where I 
have had an opportunity of examining it, seems to be quite as 
fortuitous an element as, and less constant than, the sternal are. 
That bone was seen only to be developed under the combined 
expansive and contractile action of the thorax or an equivalent 
force ; and therefore its homologue is not to be looked for in con- 
nexion with an organ of such fixed character as the spinal column, 
But separated bones for the neural spine unquestionably occur, 
and seem rather to owe their existence to the spinalis dorsi 
muscle and the supraspinal ligament. 
It has been already remarked that in certain ribs of some 
animals, as the buffalo and rhinoceros, there are well-marked 
epiphyses at the ends. Now I conclude from this, that just as 
these ribs behave themselves like separate bones in this circum- 
stance, so we are justified in believing that, hke the centra and 
limb-bones, they would have produced epiphyses in any other 
direction if the forces had favoured it ; and, indeed, the lateral 
processes of the ribs of birds may be cited as examples of such a 
modification. And it is quite possible to explain the formation 
of the Chelonian carapace by regarding the plates as external 
epiphysial overgrowths of the vertebral elements. And I sup- 
pose that the neural arches do not develope such structures be- 
tween each other only because, owing to the weakness of at- 
tachment to the centrum and the absence of ligaments and 
muscles of sufficient power, the strain was never great enough 
to produce active ossification and the vibrating tension in which 
