THE CELL 



13 



cover half blue and half red, and exposed to sunlight, are disturbed by 

 blue light, and actively move till they finally come to rest under the 

 red. This exciting effect of blue light acts on the skin and produces 

 its effect even in blind animals. It is the most refrangible rays of 

 the spectrum, the so-called ultra-violet rays, which have the marked 

 effect on living matter. These rays produce in us sunburn, followed 

 by a protective pigmentation of the skin. They act as a bactericide, 

 e.g., the tubercle bacillus is killed by light and the powerful arc light 

 (the Finsen light) is employed to cure lupus. 

 The exclusion of all but red and yellow rays 

 from the sick-room is said to prevent the sup- 

 puration of the eruption in small-pox. The 

 black man is protected from the ultra-violet 

 rays by his pigment. Similar but more powerful 

 effects are produced by mercury vapour lamps 

 enclosed in quartz. The light from these lamps 

 is particularly rich in ultra-violet rays, since the 

 spe3trum of mercury vapour contains many 

 bright lines in the ultra-violet region of the 

 spectrum, and quartz, unlike glass, is easily 

 transparent to the rays (Fig. 10). 



As a rule cells are small in size, some few 

 thousandths of a millimetre in diameter. Occa- 

 sionally for example, the egg of a bird the 

 cell is macroscopic in size, owing to the large 

 amount of vegetative or nutritive cytoplasm 

 present. 



The shape varies more than the size. In 

 the various tissues it becomes modified to 

 almost any shape flat discs, cubes, hexagons, 

 rods, or branching forms. 



There is an individuality of the cells of the 

 multicellular organism. Their power to survive 

 removal from the body is very great; thus, the 

 sperm of the drone is received by the queen bee 

 in her nuptial flight, and remains active for the 

 rest of her life in the receptaculum seminis. In 

 the cloacal sac of the female salamander the 

 sperm is retained active for two years after 

 copulation. The bat is wed in autumn, and be- 

 comes pregnant after her winter sleep. Living spermatozoa have 

 been found eleven days after excision in the excised testicles of guinea- 

 pigs kept at C. Living human spermatozoa have been found in the 

 uterus eight and a half days after cohabitation. The leucocytes of 

 the frog showed amoeboid movements after being kept three weeks 

 in a moist chamber. Dog's blood kept ten days on ice has been 

 successfully transferred into a dog. Movement of ciliated cells has 

 been observed in a tumour eighteen days after its removal from the nose. 

 Pieces of the mucous membrane of the frog's mouth put in the dorsal 



FIG. 10. LESIONS 

 PRODUCED BY THE 

 ULTRA VIOLET 

 RAYS ACTING UPON 

 THE RABBIT'S EAR 

 SCREENED BY A 

 PIECE OF BLACK 

 CARDBOARD, FROM 

 WHICH THE DESIG.V 

 AND LETTERS WERE 

 CUT OUT. (After 

 V. Henri.) 



