402 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 2 



the skin to visible light as chlorophyll does the plant. Patients who 

 failed to develop pigment have responded favorably to treatment,*® 

 just as patients who developed a strong pigment often failed to heal. 

 The develoi^ment of ultra-violet pigmentation is independent of that 

 of tolerance, and ultra-violet tolerance is possible without pigmenta- 

 tion. A suspension of melanin has been shown to protect the frog's 

 mesentery fi-om ultra-violet rays. 



Primary melanin deposit is in the basal cells of the malpighian 

 layer; and the greatest absorption in this layer, according to Baehem 

 and Reed, occurs at 300 and at 250 millimicrons. Therefore, pigment 

 production must be due mainly to these spectral regions; and it 

 appears to be the expression of a local response to the irritation of 

 the prickle cells, which lie immediately above the basal cells where 

 the pigment is deposited. In addition to the irritation, a mother 

 substance (dioxyphenylalanine or tyrosine or some closely related 

 compound) and an enz3ane (an intracellular oxidase called dopa- 

 oxidase) are necessary. 



Pigment undoubtedly is in some way correlated with increased 

 tolerance to irradiation. It not only absorbs but radiates energy. 

 It increases absorption to the yellow and green and so suggests an 

 adaptation to sunlight (which has its greatest intensity in this 

 region). It may sensitize tissues to long rays and so shift the effec- 

 tive threshold toward the red region ; but there is no evidence of its 

 changing short lethal rays to long nonlethal ones, as Rollier has 

 suggested. 



Of the rays that produce pigment readily, a fairly large percent- 

 age has been found to penetrate beyond the pigment-bearing cells. 

 Pigment cells lying beneath prickle cells can not account for the 

 increased tolerance of the latter to subsequent irradiation, unless 

 melanin may cause systemic effects that allow for this. Chemical 

 change produced in the corneal layer may also be a factor.'*^ 



DOSAGE 



Curative results in light therapy may be brought about without 

 the production of marked cutaneous burn, and even the first degree 

 of redness need not be produced. In treatment of rickets exposures 

 with the quartz mercury arc as small as five minutes anteriorly and 

 posteriorly twice weekly have proved curative. The skin offers a 

 vast field of living cells, which are exposed to stimulation or injury 

 in many ways, and such an effort may provoke immunizing or harm- 

 ful effects. Certain regions of ultra-violet between 313 and 250 

 millimicrons produce skin erythema ; about 69 per cent of the light 



« Reyn, A., Die Flnsenbehandlung, Berlin, H. Mousser, 1913. 

 *» See footnote 12. 



