Elements of animal physiology, chiefly human . 01^ THE EYE-BALL. 169 381. The Bright Spot of Sommering, macula lutea, oryellow spot, is a round, yellowish, elevated spot, about•Jj- of an inch in diameter, situated in the centre of theback of the eye in the axis of vision, and about -^-^ of aninch outside of the hlind spot. Its summit contains alittle pit or depression termed the fovea centralis. It isthe seat of most acute vision, yet it has no nerve fibresfrom the optic nerve, but it is full of close-set cones, andcontains nerve corpuscles. 382. The Duration of the Impression of Light on ther
Elements of animal physiology, chiefly human . 01^ THE EYE-BALL. 169 381. The Bright Spot of Sommering, macula lutea, oryellow spot, is a round, yellowish, elevated spot, about•Jj- of an inch in diameter, situated in the centre of theback of the eye in the axis of vision, and about -^-^ of aninch outside of the hlind spot. Its summit contains alittle pit or depression termed the fovea centralis. It isthe seat of most acute vision, yet it has no nerve fibresfrom the optic nerve, but it is full of close-set cones, andcontains nerve corpuscles. 382. The Duration of the Impression of Light on theretina is about J of a second. If, therefore, a lightedstick be rapidly moved round in a circle so that it shallreturn to the point from which it started in less than |- ofa second, it will be seen as though it were a luminouscircle. The appearance of the firework termed the Catherine wheel, and of the pictures in a zoetrope, aredue to this cause. Saperior obliqueAluscie. Superior KeetusMuscle. Interior BectusMuscle. Left Optic External RectusMuscle. Optic Commissure. ^^Motor-oculitNerve). Fig. 79. The Muscles of the Eye-Ball. 383. The Muscles of the Eye-Ball by which the eyes arerolled about and axis of vision is moved in any givendirection, consist of the four recti (straight) muscles by 170 ANIMAL PHYSIOLOGY. which the eye is rolled inwards, outwards, upwards, anddownwards, the superior oblique pulley or trochlearismuscle, and the inferior oblique muscle by which the eye isrolled on its axis at the same time that it is pulled inwardand forward. The two oblique muscles are attached alittle behind the centre on the outer side of the eye-ball,and thus give it their peculiar movement. The tendonof the trochlearis muscle passes through a tendinouspulley-like loop, and bends downwards so as to act onthe eye-ball like a cord from a pulley. The followingare the names and functions of the muscles:— 1. Superior rectus {attolois) muscle, pulls the eye-ball upwards. 8.
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