Eeceptoes, Neurons, and Effectors 129 



ways. Thus, for example, there has been supposed to be a ' writing 

 center ' which controls the muscles of the arm and hand in movements of 

 writing words ; a control distinct from that exercised over the same 

 muscles by other centers for other purposes. Centers for respiration, and 

 for vasomotor control have been located. These various centers are fig- 

 ured as possessing a sort of spontaneity or intelligence, so that they 

 simply need to be stimulated from other centers in order to operate the 

 mechanism under their care. It is not probable that the centers possess 

 any great degree of functional independence, and the view should be 

 avoided. 



2. The Theory of the Center as a Distributing or Collecting Organ. 



The term ' center ' ought by rights to be abandoned altogether ; but if 

 used at all it is properly employed to designate a nucleus, ganglion, or 

 other group of cells from which impulses may be sent forward in several 

 divergent lines (sensory center) or into which impulses are collected from 

 several proximal sources (motor center). 



The afferent neurons of the spinal system form chains along which the 

 impulse is passed from neuron to neuron. Some of these chains ultimately 

 reach the cerebral cortex. Some afferent chains possibly have not direct 

 connection with the cortex. The synaptic connections between two serially 

 contiguous neurons occur usually in regions of the cord or brain-stem 

 where lie the cell-bodies of the second of the two neurons. Such groups 

 are in the various nuclei of the brain-stem, and in Clark's column of the 

 cord. If one of these nuclei is a simple relay station, passing the impulse 

 always on over the same route, it could not be called a center. But, if 

 from a given nucleus, a current received from a peripheral neuron can be 

 transmitted either towards the cortex, or to a certain motor nucleus di- 

 rectly, the nucleus in question is a center. So with the nuclei in the effer- 

 ent chains. If they receive from several different sources, they are 

 properly called motor centers. 



The centers in the cerebral cortex are called the higher centers. The 

 centers in the brain-stem and cord are called lower centers. Each of the 

 afferent systems have one or more lower centers, although not all have 

 cortical centers. The connections of these centers and the other nuclei 

 are not completely known ; even some of the connections which have been 

 carefully studied are in dispute ; and the known details are so complicated 

 that they cannot be well introduced here. It is sufficient for the present 

 purpose to understand that the afferent and efferent neurons form an 

 enormously complicated system, in which afferent impulses can be dis- 

 tributed and collected through many synapses in the brain and cord, and 

 hence any incoming current can issue into almost any efferent channel. 



