682 The Outline of Science 



muscle of our limbs, which is under the direct control of the will, 

 the cells are enormous, with many nuclei, and the microscope 

 shows that the substance of the cells is cross-striated. The alterna- 

 tion of dark and light bands which gives this appearanct is, in 

 some way as yet unknown, necessary for rapid contraction, and it 

 is best developed in such muscles as those of insects' wings, which 

 vibrate with almost inconceivable rapidity. 



The cells of the outer skin, or epidermis, have a singular 

 fate they are continually being sacrificed for the good of the 

 body as a whole. The lower layers of the epidermis consist of 

 rounded cells which are rapidly multiplying; as the cells thus 

 produced approach the surface they become flattened and, finally, 

 converted into horny material, which eventually flakes off as the 

 so-called scarf-skin. The continual shedding of this usually 

 passes unheeded, but becomes at once evident by the masses of 

 it which accumulate when a bandage is left on for long. By 

 this means the skin continually renews itself, and the soiled and 

 bruised outer layers are shed and replaced by new cells from 

 below. A similar conversion of cells from horny material takes 

 place in our nails and hair; but here the horny structures are 

 relatively permanent. 



The Brain-Cells 



The brain too, the organ of mind itself, is composed of cells. 

 The cells of the brain and spinal cord undergo, perhaps, the most 

 remarkable development of all. Originally, in the embryo, they 

 are of simple and rounded form and like other cells ; they develop 

 outgrowths, usually two in number, which often grow to great 

 lengths and may branch repeatedly. Of the finest terminal 

 branches, some come into contact with the cells of muscles and 

 glands, others with the cells of sense-organs like the eye and ear 

 or the touch-spots in the skin, others with similar outgrowths of 

 other nerve-cells. In this way every organ in the body is linked 

 up with every other, through the mediation of a living telephone 



