The histology of the eye / by John Whitaker Hulke.

  • John Hulke
Date:
[1869]
Catalogue details

Licence: Public Domain Mark

Credit: The histology of the eye / by John Whitaker Hulke. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    far as their line of union with the appendages. It completely insulates the shafts, and would have the effect of absorhing any pencil of hght which, making a relatively small incident angle, might escape laterally outwards through the shaft-wall, and in this way the escaped pencil would be prevented from entering a neigh- bouring shaft. In mammals, the greater slenderness of the shaits probably renders such a provision imnecessary, because the incident pencil, to enter the shaft, must nearly coincide with its axis; and, as regards the side of the shaft, the angle of incidence would be so large that the pencil would probably be totally reflected. The inner ends of the rods and cones pass through apertures in the connective membrane, called the membrana Hmitans externa retm^, and are produced mwards amongst the outer granules as slender bands or fibres. The membrana hmitans externa is the sharp, hard line, always perceptible in vertical sections between the bacillary and the outer granule layers. . • -i That the rods and cones are the percipient elements in tiie^ retina is now universaUy received, so that it needs hardly be men. tioned; but it may be well to adduce the chief considerations on which this presumption rests. First, they alone of all the retmal tissues are so arranged as to be capable of receiving separate and distinct stimuh from smaU incident pencils of hght. _ Next, their absence entads absence of perception. Mariotte s experiment proves this as regards the optic nerve disc, and the mcrease of the size oi the blind-spot m myopia from posterior staphyloma, proportionately to the extent of the wHte atrophic crescent—a fact which is easily roughly verified—is another proof of the same thmg; because here together with the disappearance of the choroidal epithelium and chorio-capillaris, I have had opportunities of provmg microscopi- cally the absence of the cones and rods. . . ^ j ^ x When we endeavour to press our inquuries farther, and try to ascertain what may be the respective functions of the outer and the inner segment of the rods and cones, and m what respect tJie fanctions of the rods and cones differ, we meet with difficulties which have yet to be overcome. _ ^^ i- -u e As regards the first part of this mquiry, the high refractive index of the shafts, and their insulation by a coat ot pigment m many animals, points to a physical optical .oZ^; while the asso- ciation of a nucleus (an outer granule) with the appendage, suggests a more vital dynamical share. If this be so, then the jimction between the shaft and appendage marks the line where so to say, the physical vibrations of hght are converted mto nerve-force Towards the solution of the second pomt of the mqmry, Schultze contributes the important fact that ^octol mamma s,^^^^^^^^^ mouse, bat, hedgehog, have no cones; and that m owls, they want