106 SENSORY AND MOTOR NERVES. [BOOK i. 



X readily be affected by all changes in the world without, Fig. 16 C. 

 And just as a primary motor nerve arises as a retained thread of 

 communication between a sensitive cell and its muscular process, 

 so a primary sensory nerve may be conceived of as arising as a 

 thread of communication between an eminently sensitive cell and 

 its twin the eminently automatic or central cell. By this arrange- 

 ment the sensitive cell, relieved of the heavy burden of spontaneous 

 action, is enabled to devote itself with greater vigour to the re- 

 ception of external influences ; while the automatic cell, no longer 

 hampered by the physical necessities of being which are imposed 

 on the superficial cell, exposed as this is to every wind and wave, 

 but secure in its internal retreat, is able with similar increased 

 energy, to devote itself either to the production of spontaneous 

 impulses, or to profoundly modifying the impulses which it receives 

 from the sensitive cell. Naturally the muscular process or muscu- 

 lar fibre would on the splitting of the original single cell remain in 

 connection with the more eminently automatic. We thus arrive 

 at that triple fundamental arrangement of a nervous system, in 

 its simplest form, viz. a sensitive cell on the surface of the body 

 connected by means of a sensory nerve with the internal automatic 

 central nervous cell, which in turn is connected by means of a 

 motor nerve with the muscular fibre-cell. 



We have already seen that the physiology of the motor nerve 

 cannot without inconvenience be separated from that of the mus- 

 cular fibre. In the same way the physiology of the sensory nerve 

 cannot well be separated from those modifications of superficial 

 sensitive cells which constitute the organs of sense. We may add 

 that the special physiology of the central nervous cells can only 

 profitably be studied in connection with the sensory organs. In 

 the present chapter, therefore, we purpose to confine ourselves to 

 the consideration of the simplest and most general properties of 

 the central nervous cells. 



These are arranged in the vertebrate body in two great systems : 

 the cerebro-spinal axis, and the various ganglia scattered over the 

 body ; we shall deal with such properties only as are more or less 

 common to the two systems. We may premise that as far as our 

 knowledge at present goes, the processes which are concerned in 

 the propagation of nervous impulses along a sensory nerve-trunk 

 are identical with those which take place in a motor nerve-trunk. 

 The phenomena of the natural nerve current, of the currents of 

 action during the passage of an impulse and of electrotons (and 

 these facts mark out, as we have seen, the limits of our information 

 on this matter,) are exactly the same, whether the piece of nerve- 

 trunk experimented on be a mixed nerve-trunk, or an almost 

 purely motor, or an almost purely sensory nerve-trunk, or an an- 

 terior or posterior nerve-root, or the special sensory nerve of a 

 particular sense, such as the optic nerve. In both sensory and 

 motor nerves the changes accompanying a nervous impulse are 

 transmitted equally well in both directions. 



