. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. AVES. 307 semilunar tendinous sheath, having no express or fixed insertion. The second muscle, called Pyramidalis tiictitant'is, (h, Jig. 139,) arises from the sclerotica from the lower and nasal side of the eye-ball; its fibres con- verge as they pass to the upper side of the optic nerve, and there terminate in a small round tendon, which glides through the pulley at the free margin of the quadratus, and wind- ing round the optic nerve, passes along a cellu- lar sheath at the lower part of the sclerotica, and is inserte


. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. AVES. 307 semilunar tendinous sheath, having no express or fixed insertion. The second muscle, called Pyramidalis tiictitant'is, (h, Jig. 139,) arises from the sclerotica from the lower and nasal side of the eye-ball; its fibres con- verge as they pass to the upper side of the optic nerve, and there terminate in a small round tendon, which glides through the pulley at the free margin of the quadratus, and wind- ing round the optic nerve, passes along a cellu- lar sheath at the lower part of the sclerotica, and is inserted into the lower part of the mar- gin of the third eye-lid, along which it is continued for some distance, and is gradually lost. By the simultaneous action of the two mus- cles, the membrana nictitans is drawn forcibly outwards and with an oblique inclination down- wards over the anterior part of the eye.* The tendon of the pyramidalis gains the due direc- tion for that action by winding round the optic nerve, and it is restrained from pressing upon that nerve during the action of the pyramidalis muscle by the counteracting force of the qua- dratus, which thus augments the power of the antagonist muscle, while it obviates any incon- venience from pressure on the optic nerve, which its peculiar disposition in relation to that part would otherwise occasion. To examine this singular and beautiful me- chanism, it is necessary to remove the muscles of the eye-ball, especially the rectl. Lachrymal Organs.—-There are two glands which secrete a fluid to lubricate the ball of the eye, and facilitate the movements of the eye-lids ; one of these relates more especially to the movements of the nictitating membrane, and is called from its discoverer the Harderian Gland; the other corresponds to the ordinary Glandida lachrymalis. Fig. 140. 5*. * This oblique motion is most remarkable in the Owls, in which the nictitating membrane is ac- companied by the upper eye-lid in its sweeping move


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