Abstract
Eyeball structures such as the lens diameter (LD) and axial length are generally assumed to be highly correlated with optically meaningful parameters. However, these optical constraints on eyeball macroanatomy have never been tested explicitly. Tradeoffs between benefits of improved visual performance and cost of adaptation from an increase of tissue production predict that when eyeball size increases, optical parameters such as posterior nodal distance and maximum entrance pupil diameter should increase isometrically with eyeball axial length and LD, respectively. Here I show quantitatively that the interspecific allometry of the avian eye largely follows this predicted isometry. Additionally, I elaborate a method to estimate optically significant eyeball softโtissue dimensions from scleral ring and orbit morphology based on analyses of interspecific allometry in Aves. The stringent correlations between avian eyeball morphology and optical function render this system ideal for the analysis of formโfunction relationships and allow for an accurate estimate of optically significant eyeball softโtissue dimensions such as diameter, axial length, and LD in fossil species. J. Morphol. 2009. ยฉ 2009 WileyโLiss, Inc.