THE SPINAL CORD 557 



ism is supposed to exist when different areas in the central nervous 

 system are connected by commissural, or association fibres. The bipolar 

 cells of the ganglia on the posterior roots of the spinal nerves and of 

 the sensory cranial nerves, however, seem to be merely enlargements in 

 the course of the nerve-fibres. 



According to this theory of nerve-conduction, impulses are taken up 

 by the dendrites of the nerve-cells and transmitted onward through their 

 neurites ; and this may involve the passage of the current through a 

 number of so-called cell-stations. The cells send out impulses through 

 their neurites and receive them through the dendrites, the same mechan- 

 ism existing for both centripetal and centrifugal conduction, and " de- 

 lays " always occurring at the cell-stations. The current, therefore, can 

 move only in one direction, this depending on the arrangement of the 

 arborizations of the neurites and of the dendrites. Finally, therefore, 

 the sensory cells of the cerebral cortex receive impressions through 

 their dendrites. If it is assumed that neurites in their course may con- 

 duct in both directions, and some experiments favor this view, as a 

 logical consequence the synapses must act as valves ; so that a centrifu- 

 gal current can not pass in the opposite direction beyond a synapse, 

 the same being true of a centripetal current. 



While the theory of the action of synapses explains many heretofore 

 obscure phenomena of nerve-conduction and is extremely probable, it 

 should be received as such and not as a matter of positive and absolute 

 demonstration. There are certain properties and functions of nerve- 

 cells, connected with trophic action, degenerations and associations of 

 nerve-centres, that still are imperfectly understood. 



Relations of the Posterior White Columns of the Cord to Muscular 

 Coordination. It was noticed by Todd, many years ago (1839-1847), 

 in cases of that peculiar form of muscular mcoordination now known as 

 locomotor ataxia, that the posterior white columns of the cord were 

 diseased. Reasoning from this fact, Todd made the following state- 

 ment in regard to the office of these columns : 



" I have long been impressed with the opinion, that the office of the 

 posterior columns of the spinal cord is very different from any yet 

 assigned to them. They may be in part commissural between the sev- 

 eral segments of the cord, serving to unite them and harmonize them 

 in their various actions, and in part subservient to the function of the 

 cerebellum in regulating and coordinating the movements necessary for 

 perfect locomotion." 



The view thus early advanced by Todd has been sustained by experi- 

 ments on living animals. If the posterior columns are completely divided 

 by two or three sections made at intervals of about three-fourths of an 



