. A text-book of human physiology . pointhigher than that to which the other is directed at the same time, and the twocannot be made to diverge. Theoretically, by appropriately combined action of its six muscles, each eyecan be turned in any direction and rotated on any axis; but the actual movements are fcAv in comparisonwith those theoretically possible. Ingeneral we may say that only move-ments with the lines of vision paral-lel or symmetrically converged—i. e.,directed toward a point in the midline—are possible. Convergence ofthe lines of vision toward a point notin the mid line is always
. A text-book of human physiology . pointhigher than that to which the other is directed at the same time, and the twocannot be made to diverge. Theoretically, by appropriately combined action of its six muscles, each eyecan be turned in any direction and rotated on any axis; but the actual movements are fcAv in comparisonwith those theoretically possible. Ingeneral we may say that only move-ments with the lines of vision paral-lel or symmetrically converged—i. e.,directed toward a point in the midline—are possible. Convergence ofthe lines of vision toward a point notin the mid line is always associatedwith great effort, and as a rule isobviated by moving the head andthus avoiding, as we are always in-clined to do, extreme movements ofthe eyes. This limitation of the eye move-ments is of very great importance forvisual perceptions; for the connectionbetween the retinal images and the po-sition of the eyes is thereby renderedmore constant than would be the case if all theoretically possible movementswere carried Fig. 242.—The field of vision projected on adistant plane perpendicular to the line ofvision, after Hering. § 2. SIGNIFICANCE OF EYE MOVEMENTS FOR THE OUTWARDPROJECTION OF VISUAL PERCEPTIONS It is evident from the optical principles of the eye that the imagesthrown on the retina by refraction of light are always reversed, and yet wealways see the objects to which the images correspond right side up. Theexplanation of this phenomenon has been much discussed, and yet is all verysimple. The newborn child sees, but understands nothing of what it sees. In-cluded in the knowledge which the child gains by experience with the senseof sight is the knowledge of the position of things. But this knowledge thechild does not obtain by the sense of sight alone; the bodily movements playa determining part as well. When the child looks at its nurse the image isupside down on the retina. But if it should wish to touch the nurses headwith its hand, it must move its
Size: 1555px × 1607px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, booksubjectphysiology, bookyear1
