60 BIOLOGY: GENERAL AND MEDICAL 



result in injurious effects, known as sunburn, characterized 

 by redness (hyperaemia) and the formation of blisters 

 (vesication) and followed by loss of the superficial 

 layers of the epiderm and increased pigmentation of the 

 deeper layers, usually appearing as a uniform bronzing 

 of the skin, though in certain individuals, mostly in 

 those of fair complexions, the pigment may collect in 

 small dark spots (freckles). 



Continued exposure to the sun leads to deep bronzing 

 and may explain why the races of men inhabiting those 

 portions of the earth where the rays of the sun are most 

 intense are uniformly darker in complexion than those 

 of less sunny climes. 



Concentration of the sun's rays by lenses' results in 

 intensification of both heat and light rays, the former 

 being intensely destructive to life. If, however, the 

 concentrated rays are passed through cooling apparatus 

 so as to be deprived of the heat, it is found that the light 

 rays are also destructive to cell life. Finsen has devised 

 a method of destroying certain tumors (squamous cell 

 carcinoma) by exposing them to such concentrated light 

 rays, the abnormal cells of the tumor seeming to be less 

 able to endure their effects than those of the normal 

 tissue cells among which they grow. 



GALVANOTROPISM OR RESPONSE TO ELECTRICAL 

 STIMULI. 



Electrical currents of high intensity are destructive 

 to all forms of life through chemical and physical altera- 

 tions effected in the protoplasm. Currents too mild to 

 be destructive influence living matter but slightly, and 

 little evidence is at hand to show that stimuli of electrical 

 nature play any important r61e in the vital processes. 



Plants seem to be far less sensitive than animals to 

 the effects of electric currents. The phanerogames, in- 

 deed, show no visible electrotropic reactions, though the 

 cells when examined microscopically, as in the hairs of 



