426 THE BODY AT WORK 



of the body, the lungs, the alimentary canal, and other viscera. 

 It has no end-organs, no defined tracts in the central nervous 

 system, no definite connections with the cortex of the great 

 brain. The currents which it conducts, if they originate in the 

 visceral part of this system, have no direct effect in conscious- 

 ness ; but if they originate on the surface of the body, or in the 

 alimentary canal at the lower end of the oesophagus, or in 

 certain other situations, they co-operate with stimuli of heat, 

 cold, or traction. The critical system works in a more definite 

 way. Its impulses originate in sense-organs. Starting with a 

 certain potential, they are transmitted by the discharge of a 

 succession of linked neurones. When they reach the cortex 

 their potential is sufficiently high to evoke consciousness. 

 Their distribution in the cortex is as definite as their origin. 



Specialized sense-organs are necessary for the origin of all 

 sensations. Within the epithelium are certain cells which look 

 as if they were specialized for sensory purposes. The deeper 

 sheet, or derma, of the skin is abundantly provided with struc- 

 tures in which nerves end in the most elaborate and compli- 

 cated ways (Fig. 42). They are found especially in the 

 papillse of connective tissue, which, set in rows, form the ridges 

 that one can see at the finger-tips and in various other situa- 

 tions. All of these organs are made up of groups of epithelial 

 cells which, displaced from the epidermis, have sunk into the 

 derma, with the nerves connected with them. In their further 

 development the nervous part of the apparatus is complicated 

 by branching, the branches being thickened and usually 

 flattened into ribbons, which lie on the external surfaces of 

 the cells or between them. A more or less marked capsule 

 is provided for the organ by condensation of connective 

 tissue. 



Anyone can convince himself that the skin is not uniformly 

 sensitive. He may test it first for the minimal stimulus which 

 excites a sensation of touch. With a hair of the head it must 

 not be a very fine one cut across with scissors, and held 

 between finger and thumb at the right distance from the cut 

 end, the skin of the palm of the hand is prodded. Every here 

 and there a spot is found which is insensitive to so slight a 

 pressure. These spots are neither large nor very close together. 

 If the hairless skin of the arm between the elbow and the 



