. Animal and vegetable physiology, considered with reference to natural theology. Biology; Physiology; Plant physiology; Natural theology. VISION. 353 is found in almost every bird having extensive powers of vision.* The comparative anatomy of the eye offers, in- deed, a great number of special structures of which we do not understand the design, and which I have therefore pur- posely omitted to notice, as being foreign to the object of this treatise. In most birds the memhrana nictiians, or third eye-lid, is of considerable size, and consists of a semi-transparent fold of the conjunctiva, lyi
. Animal and vegetable physiology, considered with reference to natural theology. Biology; Physiology; Plant physiology; Natural theology. VISION. 353 is found in almost every bird having extensive powers of vision.* The comparative anatomy of the eye offers, in- deed, a great number of special structures of which we do not understand the design, and which I have therefore pur- posely omitted to notice, as being foreign to the object of this treatise. In most birds the memhrana nictiians, or third eye-lid, is of considerable size, and consists of a semi-transparent fold of the conjunctiva, lying, when not used, in the inner cor- ner of the eye, with its loose edge nearly vertical: it is re- presented at N, Fig. 434, covering half riie surface of the eye: its motion, like that of a curtain, is horizontal, and is effected by two muscles: the first of which, seen at q, in Fig. 435, is called from its shape the quadratus^ and arises from the upper and back part of the sclerotica: its fibres de- scending in a parallel course towards the optic nerve, where 433. 434 ^Zb. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Roget, Peter Mark, 1779-1869. Philadelphia, Carey, Lea & Blanchard
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