THE PHYSIOLOGY OF THE SPECIAL SENSES. 141 



A little later these cells begin to arrange themselves in groups, 

 and, neglecting the rest for our present purpose, we will confine 

 our attention to those on the surface of the sphere. They are 

 called the epiblastic cells, and form the surface of the organism 

 (Fig. 6, E). A certain number of these, having a definite posi- 

 tion, now begin to fold themselves inwards (Fig. 6, F), and finally 

 to become detached from the rest of the epiblastic cells (Fig. 6, G), 

 which close over them, leaving them inside as a ring of epiblast 

 cells. This ring of epiblast cells (which were once surface cells) 

 represents the cells from which the future nerve-cells will be 

 formed, whilst the rest of the epiblast gives rise to the surface-cells 

 of the adult body, which we call the epidermis, or skin. So, you 

 see, the antecedents of the brain and spinal cord cells were origin- 

 ally surface cells, and occupied the same position that the cells 

 forming the skin do now. By means of long filamentous pro- 

 cesses, which we call nerves, proceeding from the cells of the 

 Central Nervous System, these brain-cells join themselves to the 

 surface or epithelial cells (Fig. 6, H). In this way the central cells 

 can be informed, by a sort of telegraphic system, with what is 

 going on at the surface of the body. Thus, what we call our 

 special senses are nothing more than conscious impressions pro- 

 duced in the cells of our brains by the various external stimuli 

 acting upon the non-conscious cells of our surface and conveyed 

 up to the brain along the nerve-fibres. 



And this is true of all our different so-called Special Senses. 

 They all require for their carrying out efficiently a surface cell or 

 cells, a nerve-fibre, and a brain-cell intact. If any one of these is 

 destroyed, the power of receiving outside impressions accurately is 

 lost. The eye, the ear, the inside of the nose, and the mouth are 

 merely modifications and elaborations of these elementary nerve- 

 epithelium continuations, adapted to receiving particular kinds of 

 impressions or stimuli only, whereas the whole of the rest of the 

 body surface receives simpler and less elaborate stimuli. The 

 waves of radiant heat from the sun stimulate the epithelial cells of 

 the whole surface of the body, producing a sensation of heat ; the 

 same rays, or very closely allied ones, stimulate the epithefial cells 

 of the eye and produce a sensation of light. 



The waves of sound produced by a heavy cart passing in the 



