File:A text-book of physiology for medical students and physicians (1911) (14776542124).jpg

Identifier: textbookofphysio1911howe (find matches)
Title: A text-book of physiology for medical students and physicians
Year: 1911 (1910s)
Authors: Howell, William H. (William Henry), 1860-1945
Subjects: Physiology
Publisher: Philadelphia, London, W.B. Saunders company
Contributing Library: Columbia University Libraries
Digitizing Sponsor: Open Knowledge Commons

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in the brain the sensation of white. On theother hand, when not acted upon by light this same substance under-goes assimilatory processes that in turn set up nerve impulses whichin the brain give us a sensation of black. There are in the retina alsoa red-green and a yellow-blue substance. The former when actedupon by the longer rays undergoes disassimilation and gives asensation of red, while the shorter waves cause assimilation andproduce a sensation of green. A similar assumption is made forthe yellow-blue substance. The essence of the theory may be stated,therefore, in tabular form, as follows *: Photochemical Substance. ■p, j ( Disassimi Red-green \ Assimilat Yellow-blue Retinal Process. Sensation. Disassimilation = red ion = green / Disassimilation = yellow l Assimilation = blue Tm--.i. iii / Disassimilation = white White-black ( Assimilation = black It will be observed that the theory gives an independent ob-jective cause for the sensations of white, black, and yellow, and in
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Fig. 154.—Schema to illustrate the Hering theory of color vision.—(Foster.) Thecurves indicate the relative intensities of stimulation of the three color substances by dif-ferent parts of the spectrum. Ordinates above the axis, X-X, indicate catabolic changes(disassimilation), those below anabolic changes (assimilation). Curve a represents theconditions for the black-white substance. It is stimulated by all the rays of the visiblespectrum with maximum intensity in the yellow. Curve c represents the red-green sub-stance, the longer wave lengths causing disassimilation (red), the shorter ones assimilation(green). Curve b gives the conditions for the yellow-blue substance. this respect satisfies the objections made on this score to the Young-Helmholtz theory. It fits better, also, the facts of partial and totalcolor blindness. In the latter condition one may assume, in terms of * For discussion of color theories see Calkins, Archiv f. Physiologie,1902, suppl. volume, p. 244; also Gre

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