THE CENTRAL NERVOUS SYSTEM. 



235 



to the periphery, centrifugal impulses, form the axis-cylinders of 

 some of the nerves. The axis-cylinders of those nerves which 

 convey impulses from the periphery toward the nervous centres, 



FIG. 217. 



II 



Sketch illustrating the composition of neurons. I, a neuron transmitting centrifugal 

 impulses. II, a neuron receiving and transmitting centripetal impulses. Ill, a neuron, 

 the function of which is supposed to be the distribution of impulses within the nerve- 

 centre in which it is situated, a, ganglion-cell ; 6, dendrite ; c, teledendrites ; d, neurite ; 

 e, collaterals ; /, teleneurites. In II the body c represents some sensory organ imparting 

 nervous impulses to the teledendrites of a sensory nerve. The nervous filament g is a 

 neurite, presumably derived from the sympathetic nervous system, leading to teleneu- 

 rites applied to a ganglion-cell, a, of a posterior spinal ganglion. The portion h of the 

 "nerve" springing from that cell is regarded as a portion of the cell itself. In the 

 embryonic condition the dendrite and neurite both spring directly and separately from 

 the body of the cell, the portion h being a subsequent development, i, endothelial 

 envelope surrounding the ganglion-cell. Ill represents a ganglion-cell, apparently devoid 

 of distinct dendrites, but having numerous processes that at first appear protoplasmic, 

 but soon assume the characters of neurites. These cells are found in the retina and 

 olfactory bulb, and have been termed spongioblasts, cellulas amacrinas, and parareticu- 

 lar cells. It is thought that nervous stimuli are received directly by the cytoplasm of 

 the cell, without the intermediation of dendrites. x represents the omission of a portion 

 of a fibre. The arrows indicate the directions taken by nervous impulses. 



centripetal stimuli, may be the dendrites connected with ganglion- 

 cells in or near those centres ; c. </., in the posterior root-ganglia of 

 the spinal nerves, or they may be the neurites springing from 



