424 



THE BODY AT WORK 



a tiny beetle injures the surface of the cornea by scratching 

 the epithelial cells with its horny wings and legs, the ruptured 

 nerve-filaments convey to consciousness impulses, or, as we 

 prefer to express it, an influence which is felt as pain. But 

 even the pain caused by injury to the cornea is trifling as com- 

 pared with that which originates in the under-sides of the lids, 

 where not only is the epithelium supplied with branching 

 nerve-twigs, but specialized organs of touch are present to 

 localize the seat of injury. Everywhere the epithelium 



FIG. 41. VERTICAL SECTION OF THE EPITHELIUM WHICH COVERS THE SURFACE OF THE CORNEA, 

 AND OF A SMALL PORTION OF THE CORNEAX SUBSTANCE, HIGHLY MAGNIFIED. 



The black lines are naked nerve-fibres (stained with chloride of gold), which are distributed 

 amongst the cells of the more superficial strata of the epithelium in very great abundance. 

 The corneal substance is composed of sheets of transparent fibres with intervening cells. 

 As the fibres of the several sheets cross one another at various angles, they are cut, some 

 transversely, others in the direction of their length. 



covering the surface of the body is so abundantly supplied 

 that a successful staining of nerve-filaments induces one to 

 think that every epithelial cell has its nervous affiliation. 

 These are the nerves of common sensation, if we retain the 

 term ; but sensation so common, so obscure, so little differen- 

 tiated that we know no more about it than we know about the 

 air which envelops our hands and faces on a warm, windless 

 day. Yet the air, when it moves, gives rise to a dim, broad, 

 generalized sensation, which may be focussed into definiteness 

 by a sensitive nerve. 



An observer who has devoted himself for many years to the 

 investigation of skin-sensations, and especially of the " re- 

 ferred pains " which are due to diseases of the viscera, recently 

 caused the large cutaneous nerve which supplies the thumb 



