Identifier: textbookofphysio1916howe (find matches)
Title: A text-book of physiology : for medical students and physicians
Year: 1916 (1910s)
Authors: Howell, William H. (William Henry), 1860-1945
Subjects: Physiology Medicine
Publisher: Philadelphia and London : W. B. Saunders Company
Contributing Library: Yale University, Cushing/Whitney Medical Library
Digitizing Sponsor: Open Knowledge Commons and Yale University, Cushing/Whitney Medical Library

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ibers, is found within the eyeball, lyingbetween the choroid and the sclerotic coat at the point at which thesclerotic passes into the cornea and the choroid falls into the ciliary* Helmholtz, Handbuch der physiologischen Optik, second edition, 1896. DIOPTRICS OF THE EYE. 315 processes. Some of its fibers take a more or less circular directionaround the eyeball, resembling thus a sphincter muscle, while otherstake a radial direction in the planes of the meridians of the eye andhave their insertion in the choroid coat (Fig. 131). When thismuscle contracts the radial fibers especially will pull forward thechoroid coat. The effect of this change in the choroid is to loosenthe pull of the suspensory ligament (zonula Zinnii) on the lens andthis organ then bulges forward by its own elasticity. The theoryassumes that in a condition of rest the suspensory ligament, whichruns from the ciliary processes to the capsule of the lens, exerts a Ciliary Borderprocess, of iris. Ciliary muscle. ljens.-
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Choroidcoat. Ora serrata. Fig. 131.—Meridional section of eyeball after removal of sclerotic coat, cornea, and iris,to show the position of the ciliary muscle.—(Schultze.) tension upon the lens which keeps it flattened, particularly alongits anterior surface, since the ligament is attached more to this side.When this tension is relieved indirectly by the contraction of theciliary muscle the elasticity of the lens, or rather of the capsule ofthe lens, causes it to assume a more spherical shape along its anteriorsurface, and the amount of this change is proportional to theextent of contraction of the muscle. Other theories have beenproposed to explain the way in which the contraction of the ciliarymuscle effects a change in the curvature of the lens,* but none isso simple and, on the whole, so satisfactory as the one suggestedbv Helmholtz. It is interesting to note that in fishes accommodation is effected in adifferent way, namely, by movements of the lens forward and backward.In th

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