NEURONS 223 



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 T 2Vo to TCTO * n> ^he 

 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 con- 

 tinue for some distance after the sheath is lost, as in the white 

 substance of the encephalon, but never penetrates the gray sub- 

 stance proper. Every nerve fiber is connected with a cell by 

 that cell's axis-cylinder prolongation. 



Certain retrograde changes take place in the neurons in old 

 age morphological changes agreeing with the physiological 

 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 proto- 

 plasm 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 dimin- 

 ished 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 ganglion. Its end arborizations 

 simply interlace with the dendrites 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 motor 

 area of the brain pass themselves out as parts of the anterior roots. 



