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 

 ligaments 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 hgament, the capsular liga- 

 ment, and the ligamenta subilava; 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 arc. 

 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, like 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 ta 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 



