THE SENSORI-MOTOR SYSTEM 27 



protoplasm containing a nucleus. From one side of such a cell 

 there arise one or more prolongations of the protoplasm, and these 

 branch repeatedly to form a plant-like growth an arborisation 

 which is known as the dendritic system of the cell. From some 

 other part of the cell there arises a delicate filament which does 

 not usually branch, but is prolonged out into a long thread, the 

 nerve fibre. As Fig. 8 shows, this fibre, which is called the 

 axon of the cell, becomes invested in a sheath, and between the 

 sheath and the axon there is a sort of packing, the medulla. The 

 axon may be very long; thus the cells of the grey matter of the 

 spinal cord send out axons into the sciatic nerve, and some of 

 these may be about 3 feet in length, though the nerve cell itself 

 is only about ^-Q-Q to ^ inch in diameter. The axons, of 



STIMULUS 



Cell 

 Jtferve terminations 



Motor plate J$EURONE 



FIG. 9. DIAGRAM OF THE WAY IN WHICH NEURONES ARE 

 CONNECTED. 



course, constitute the nerves. At the extremity away from 

 the nerve cell the axon always breaks up into a second series of 

 dendrites, or terminations. 



The essential part of a sense organ such as, for instance, the 

 retina of the eye, or the auditory cells and hairs in the organ of 

 Corti in the internal ear, always consists of the proximal part of 

 a neurone, or even of several neurones, each of which has usually 

 a very short axon. It is always the dendrites of a nerve cell that 

 receive the stimulus, and the latter is always conducted along 

 the axon from the nerve cell to the other terminal arborisation. 

 Thus nerves invariably conduct impulses in one direction only. 



Neurones are always connected together by means of synapses. 

 At a synapse the distal dendrites of one neurone come into close 

 proximity to the proximal dendrites of another neurone. As 



