NEURONS 223 



but not actually joining, similar poles from those cells. The 

 multipolar cells in the anterior cornua of gray matter of the 

 cord are said to be larger in size and to present more poles 

 than corresponding cells in the posterior column. 



The diameter of nerve cells varies from M.250 to %oo in. 

 The nucleus is usually single, and most cells have no true 

 surrounding membrane. If a nerve fiber be followed toward 

 the center which gives it origin it will be found first to lose 

 its sheath and later its medullary substance ; this medullary 

 substance may continue for some distance after the sheath 

 is lost, as in the white substance of the encephalon, but never 

 penetrates the gray substance proper. Every nerve fiber 

 is connected with a cell by that cell's axis-cylinder prolon- 

 gation. 



Certain retrograde changes take place in the neurons in 

 old age morphological changes agreeing with the physio- 

 logical decrease in energy-producing- power at that time. 

 The cell body becomes smaller, the dendrites atrophy, and 

 the axones diminish in mass. Nerve "fatigue" can also be 

 demonstrated by the microscope. The nuclei of the sheath 

 are flattened, the protoplasm is shrunken and vacuolated 

 and the nucleus is crenated. The quantity and quality of 

 the food may be perfect, but the power of the cell to utilize 

 it is impaired, and this means diminished physiological 

 power. 



Communication Between Different Neurons. Every neu- 

 ron is anatomically independent of every other neuron. 

 There is no actual joining of fibers or dendrites simply an 

 interlacement of the end arborizations. This is illustrated 

 in Figs. 62 and 63. In the latter the afferent fiber is joined 

 to no cell except G, one of the cells of the spinal root gang- 

 lion. Its end arborizations simply interlace with the dend- 

 rites of the motor cell M. C. and cause it to send out an 

 efferent impulse to the muscle M. 



Furthermore, there are frequent relays in the transmission 

 of nerve messages. By no means do all the fibers from the 



