File:Physiology - a manual for students and practitioners (1892) (14775835771).jpg

Identifier: physiologymanual00mann (find matches)
Title: Physiology : a manual for students and practitioners
Year: 1892 (1890s)
Authors: Manning, Frederick A
Subjects: Physiology
Publisher: Philadelphia : Lea Brothers
Contributing Library: Columbia University Libraries
Digitizing Sponsor: Open Knowledge Commons

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tic nerve. Describe the retina. It consists of the nervous mechanismof the eye lying in a loose connectivetissue, and beneath it n 2>igment-Iayer.Without entering into the details ofits numerous layers, we may say thatthe fibres of the optic nerve spreadout, divested of neurilemma, in thismembrane, and communicate withganglion-cells, which are abundant.The fibres pass inward and terminatein the layer of rods and cones whichform the end-organs of the nerve. Describe the rods and cones of theretina. They are closely packed at the sur-face of the retina, the rods being themore numeroiis in most situations.The cones seem to be modifications ,. ^v^ - of the rods, and their office is essen- SfCt^Jn^^^f^^s^tially similar. The rods (Fig. 48) arestraight cylindrical bodies of a trans-parent substance, and are placed par-allel to one another and perpendicularto the surface of the eyeball. In length they are about five to seventimes the diameter of a red blood-corpuscle, and in diameter about
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Diagrammatic Section from the pos-terior Portion of the Human Ret-ina: 1, layer of rods and cones;2, layer of nuclei (Schultze). 164 THE SENSES. one-twentieth of their length. The cones are very similar, exceptthat their conical shape makes them appear to be of different cha-racter. The cones do not always reach to the same level as therods. When viewed from the retinal surface, the ends of the rodsand cones give the appearance of a fine mosaic. These organs areconnected with the subjacent nervous tissue, but the mode of theirconnection with the optic nerve is not fully understood. Considered as an apparatus, how is the eye arranged? It may be compared to the photographic camera. It contains vari-ous media for the refraction of light, and a screen at the back forreceiving the image. The refracting media are the cornea, aqueoushumor, crystalline lens, and the vitreous humor: the screen is theretina. The pigment of the retina and choroid makes the interiordark, a necessary feature in su

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