155. Again, the _re_dent--the 'tooth within tooth' of a French tracery--may be necessary, to bite its glass. But the cusp, cuspis, spiny or spearlike point of a thirteenth century illumination, is not in the least necessary to transfix the parchment. Yet do you suppose that the structural convenience of the redent entirely effaces from the mind of the designer the aesthetic characters which he seeks in the cusp? If you could for an instant imagine this, you would be undeceived by a glance either at the early redents of Amiens, fringing hollow vaults, or the late redents of Rouen, acting as crockets on the _outer_ edges of pediments. 156. Again: if you think of the tracery in its _bars_, you call the cusp a redent; but if you think of it in the _openings_, you call the apertures of it foils. Do you suppose that the thirteenth century builder thought only of the strength of the bars of his enclosure, and never of the beauty of the form he enclosed? You will find in my chapter on the Aperture, in the "Stones of Venice," full development of the aesthetic laws relating to both these forms, while you may see, in Professor Willis's 'Architecture of the Middle Ages,' a beautiful analysis of the development of tracery from the juxtaposition of aperture; and in the article 'Meneau,' just quoted of M. Violet le Duc, an equally beautiful analysis of its development from the masonry of the chassis. You may at first think that Professor Willis's analysis is inconsistent with M. Violet le Duc's. But they are no more inconsistent than the accounts of the growth of a human being would be, if given by two anatomists, of whom one had examined only the skeleton and the other only the respiratory system; and who, therefore, supposed--the first, that the animal had been made only to leap, and the other only to sing. I don't mean that either of the writers I name are absolutely thus narrow in their own views, but that, so far as inconsistency appears to exist between them, it is of that partial kind only. 157. And for the understanding of our Pisan traceries we must introduce a third element of similarly distinctive nature. We must, to press our simile a little farther, examine the growth of the animal as if it had been made neither to leap, nor to sing, but only to think. We must observe the transitional states of its nerve power; that is to say, in our window tracery we must consider not merely how its ribs are built, (or how it stands,) nor merely how its openings are shaped, or how it breathes; but also what its openings are made to light, or its shafts to receive, of picture or image. As the limbs of the building, it may be much; as the lungs of the building, more. As the _eyes_ [1] of the building, what?