224 HISTOLOGY 



LITERATURE 



CHILD, C. M. "Em bischer wenig beachtetes antennales Sinnesorgan der Insecten," etc., 



Zeits.f. Wiss. Zool., Band LVIII, 1894, pp. 478-528. 



HENSEN, V. "Uber Has Gehororgan von Locusta," Zeits. f. Wiss. Zool., Band XVI, 1866. 

 GRABER, Virus. "Die chordotonalen Sinnesorgane und das Gehor der Insecten," Arch. 



f. mik. Anat., Band XX, p. 506. 

 DENKER, A. "Zur vergleichenden Anatomic des Gehororgans der Saugertiere," Erg. 



Anat. 11. Entwick, Band IX, 1899. 

 STREETER. On the Development of the Membranous Labyrinth and the Acoustic and 



Facial Nerves in the Human Embryo. Am. Journ. of Anatomy, Vol. VI, Part 2. 

 SMITH, G. "The Middle Ear and Columella of Birds," Quart. J. Mic. Science, Vol. 



XLVIII,..i9o 4 . 

 KISHI, J. "liber den peripheren Verlauf und die Endigung des Nervus Cochleae," Arch. 



f. mik. Anat., Band LIX, 1902. 

 RETZIUS, G. "Die Endigungsweise des Gehornerven," Biol. Unters., 1892 und 1893. 



TISSUES OF LIGHT PERCEPTION 



Light is produced by short, rapid undulations of the invisible ether. 

 These movements are capable of producing chemical and physical 

 changes in living and dead matter. Their effect on ordinary animal 

 cells is not perceptible at the nerve centers. By some, they are thought 

 to be a stimulant and, in too great quality or intensity, a poison to ordi- 

 nary protoplasm. Thus the surface of most animal bodies is fitted to 

 keep the light from all underlying parts (see Chapter XIII, Pigment). 



Upon certain of the body cells, however, light does leave a definite 

 impression. There are some nerve cells that not only perceive the light 

 but are stimulated by it to send a report of the fact as an impulse to a 

 nerve center, either through their own efferent process or through com- 

 municatory nerve cells that form a path. Such cells are known as the 

 visual cells, retinula, rod-cells, etc. We shall call them the visual cells. 

 The light is given access to them through accessory tissues made trans- 

 parent for the purpose. 



The visual cell, in its specialized state, has developed a specific cell 

 organ, a peculiar, rod-like structure secreted or otherwise formed by 

 the cytoplasm and capable of being stimulated by the ether waves. It 

 is weakly developed and almost invisible in some low sight cells, while 

 in others it is larger than the cell which produced it, and so clearly dif- 

 ferentiated that it appears to be a separate structure. It is, where care- 

 ful observations have been made, laminated, and the separate plates or 

 rods of which it is composed usually lie at right angles to the light which 

 stimulates it. Its exact chemical and physical relations to the light 

 waves during stimulation are not known. It can be stimulated by other 

 factors than the light waves, as pressure and chemical activity, but it 

 gives, under these circumstances, the same impression to the brain 



