Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 76

The cornea is a thick‐walled, pressurized, partially intertwined, unidirectionally fibril‐reinforced laminate biocomposite, which imparts stiffness, strength, and extensibility to withstand both inner and outer forces that may alter its shape or integrity. A soft, fibrous connective tissue, like the cornea, usually is much stronger in the parallel versus perpendicular direction to the collagen fibrils. Consequently, the collagen fibrils are arranged into complex hierarchic structures, which give the cornea its anisotropic mechanical properties. The collagen lamellar architecture of the cornea varies dramatically between vertebrate species, with nonmammalian vertebrates exhibiting an orthogonal‐rotation arrangement with a marked increase in lamellar branching in species such that birds >> reptiles > amphibians > fish; in contrast, the mammalian species exhibit a random pattern.

Tissues are biomechanically characterized by measuring the elastic modulus, a property that defines a material's ability to resist deformation under an applied stress, which approximates its stiffness. The elastic moduli of the corneal layers as measured by atomic force microscopy have been reported in the human and the rabbit (Table 2.3); all layers of the human cornea were stiffer than those of the rabbit. This variability in corneal collagen fiber organization and matrix properties between species likely contributes to their diverse mechanical properties, and may influence indentation tonometry (estimation of intraocular pressure [IOP]).