. A text-book of comparative physiology for students and practitioners of comparative (veterinary) medicine . are recognized, the remedy should beat once applied, otherwise complications that may be to a largeextent irremediable may ensue. VISUAL SENSATIONS. We have thus far considered merely what takes place in theeye itself or the physical causes of vision, without reference tothose nervous changes which are essential to the perception of VISION. 539 an object. It is true that an image of the object is formed onthe retina, but it would be a very crude conception of nervousprocesses, indeed,
. A text-book of comparative physiology for students and practitioners of comparative (veterinary) medicine . are recognized, the remedy should beat once applied, otherwise complications that may be to a largeextent irremediable may ensue. VISUAL SENSATIONS. We have thus far considered merely what takes place in theeye itself or the physical causes of vision, without reference tothose nervous changes which are essential to the perception of VISION. 539 an object. It is true that an image of the object is formed onthe retina, but it would be a very crude conception of nervousprocesses, indeed, to assume that anything resembling thatimage were formed on the cells of the brain, not to speak ofthe superposition of images inconsistent with that clear mem-ory of objects we retain. Before an object is seen, not onlymust there be a clear image formed on the retina, but impulsesgenerated in that nerve expansion must be conducted to thebrain, and rouse in certain cells there peculiar molecular condi-tions, upon which the perception finally depends. For the sake of clearness, we may speak of the changes. Fig. 387. Fig. 388. Fig. 387.—Vertical section of retina (after H. Mflller). 1, layer of rods and cones; 2rods; 8, cones; 4, 5, 6, external granule layer; 7, interna! granule layer; !), 10 fine-ly granular gray layer; 11, layer of nerve-cells; 1:2,14, fibers of optic nerve; 13membrana hmitans. Fio. 388.—Connection of rods and cones of retina with nervous elements (after Sap-|>ey) 1,2,3, rods and cones seen from in front; 4, 5,0, side view. The rest willbe clear from the preceding figure. 540 COMPARATIVE PHYSIOLOGY. effected in the retina as sensory impressions or impulses, which,when completed by corresponding changes in the brain, developinto sensations, which are represented psychically by percep-tions ; hence, though all these have a natural connection, theymay for the moment be considered separately. It is as yetbeyond our power to explain how they are related to ea
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