THE NEEVOUS SYSTEM 333 



a more central situation. These " ganglia " were united by 

 longitudinal and transverse commissures. They received the 

 axons of the cells which lay in the clumps beneath the sense- 

 organs. They gave axons to various muscles. Such is the 

 type out of which the modern nervous system has developed : 

 two separate sense-organs and a complete nervous system for 

 each segment, the sense-organs connected with the ganglion of 

 the same side, the ganglia of the two sides bound together 

 across the middle line, and each row of sense-organs and each 

 row of ganglia united by longitudinal commissures into a chain. 

 From the nervous system as we see it now the majority of 

 these segmental sense-organs have disappeared ; but the mode 

 of formation of the cerebro-spinal ganglia shows that they are 

 the clumps of nerve-cells which lay beneath the vanished organs. 

 In the nose and the eye the grey matter retains its original 

 situation in the immediate vicinity of the receiving epithelial 

 cells as the olfactory bulb and the deeper (anterior) layers of 

 the retina. The ganglia of the auditory nerve lie within the 

 bones of the ear. Spinal ganglia are close to the spinal cord. 

 Auditory and spinal ganglia contain only the cell-bodies of the 

 first collecting neurones (sensory nerves) together with certain 

 curious bracketing cells already referred to (p. 324), all the 

 other constituents of the peripheral clumps of grey matter 

 which are found in the olfactory bulb and retina having been 

 withdrawn from the spinal ganglia into the axis of the brain 

 and spinal cord. 



The sense-organs in front of the mouth have had from the 

 beginning an immense advantage over the others as observing- 

 stations. Whereas the body-organs collected information re- 

 garding the things with which the animal came in contact, and 

 consequently specialized in touch, pressure, temperature, and, 

 in the case of fishes, sensitiveness to the chemical constitution 

 of the medium in which the animal lived, the head-organs 

 specialized in responsiveness to forces acting from a distance 

 particles suspended in air, vibrations of light, pulsations of 

 sound. Sensitiveness to touch, if it is to be useful, must be 

 widely distributed. The body-organs therefore broke into 

 scattered groups of sense-cells. Touch-spots are scattered all 

 over the surface, although they are set much closer together 

 in the areas of skin which are usually the first to come into 



