A text-book of physiology, for medical students and physicians . over the top of the verticalrod at the right at a fixed point in the middle of the semicircular bar. The extent and portion of the retina affected may be used to aid inlocating the seat of the lesion. For physiological and for clinicalpurposes it is necessary to distinguish between the central (or direct)and the peripheral (or indirect) fields of vision. The former termis meant to refer to that portion of the field which falls upon thefovea centralis; in other words, it is the projection, in any fixed 334 THE SPECIAL SENSES. posi
A text-book of physiology, for medical students and physicians . over the top of the verticalrod at the right at a fixed point in the middle of the semicircular bar. The extent and portion of the retina affected may be used to aid inlocating the seat of the lesion. For physiological and for clinicalpurposes it is necessary to distinguish between the central (or direct)and the peripheral (or indirect) fields of vision. The former termis meant to refer to that portion of the field which falls upon thefovea centralis; in other words, it is the projection, in any fixed 334 THE SPECIAL SENSES. position of the eye, of the fovea into the external world. Theperipheral field refers to the rest of the visual field involving theretina outside the fovea. As a matter of fact, all of our distinct andmost useful vision in the daytime at least is effected through thefovea. When the eye is kept fixed the small portion of the externalworld that falls upon the fovea is seen distinctly. All the rest isseen more or less indistinctly in proportion to the distance of its. Fig. —Perimeter chart to show the field of vision for a right eye when kept in a fixed position. retinal image from the fovea. In using our eyes, therefore, we keepthem continually in motion so as to bring each object, as we payespecial attention to it, in the field of central vision. The linefrom the fovea to the point looked at is designated as the line ofsight. The area of the fovea is quite small. The measurementsgiven by different observers vary somewhat, especially as in somethe measurements are estimated for the bottom of the de-ion, the fundus, and in others for the diameter from edge to edge. The average diameter is usually given as lying between ().)!and mm. bines drawn from the ends of this diameter to the nodal point of the eye subtend an angle of I degree to degrees; and therefore all objects in the external world around the line of sight whose visual angle is within this limit, iare comprised
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