64 



CEREBRAL DEVELOPMENT 



of these bucklei-a to the upper surface of the osseous skull^ 

 the separate parts necessary for the building up of the one 

 were anticipated, by many ages, in the building up of the 

 other i just as we find external arches of stone which were 

 erected two thousand years ago, constructed on the same 

 principle, and relatively of the same parts, as internal arches 

 of brick built in the present age. Doubtless, however, with 

 this mechanical necessity for correspondence of parts in the 

 formation of corresponding erections, there may have mingled 

 that regard for typical resemblance which seems so marked 

 a characteristic of the style, if I may so express myself, iil 

 which the Divine Architect gives expression to his ideas. 

 The external osseous buckler He divided after the general 

 pattern which was to be exemplified, in latter times, in the 

 divisions of the internal osseous skull ; as if in illustration 

 of that " ideal exemplar " which dwelt in his mind from 

 eternity, and on the palpable existence of which sober science 

 has based deductions identical in their scope and bearing with 

 some of the sublimest doctrines of the theologian. " The re- 

 cognition," says Professor Owen, "of an ideal exemplar for 



through the growth of the previously existing pieces, — the minute bricks 

 of my illustration, — but through the addition of new ones. Equally in 

 either case, however, that essential difference 

 between the placoid skull and the placoid verte- 

 brata to which I have referred appears to hinge 

 on the circumstance, that while the osseous nu- 

 cleus of each vertebral centrum could form, in 

 even its most complicated shape, from a single 

 point, the osseous walls of the cranium had to be 

 formed from hundreds. The accompanying dia- 

 gram serves to show after what manner the ver- 

 tebral centrum in the ray enlarges with tne 

 growth of the animal, by addition of bony mat- 

 ter external to the point in the middle, at which ossification first begins. 

 The horizontal lines indicate the lines of increment in the two internal 

 cones which each centrum comprises, and the vertical ones the lines oi 

 increment in the lateral pillars. 



SECTION OP VERTEBRAL 

 CENTRUM OF THORN- 

 BACK. 



