556 NERVOUS SYSTEM 



Sensory Paths in the Cord. The gray matter of the cord is the 

 part concerned in the conduction of sensory impressions. This fact has 

 been verified by recent experiments ; but it is thought that some of the 

 sensory conductors run in the columns of Goll. The columns of Goll, 

 however, exist only in the cervical and upper dorsal regions. 



The sensory conductors do not decussate at any particular point as 

 do the motor conductors in the crossed pyramidal tracts. The fibres 

 from the posterior roots of the spinal nerves pass to the sensory cells 

 of the posterior cornua and decussate throughout the length of the 

 cord. If the cord is divided longitudinally in the median line, there is 

 complete paralysis of sensation on both sides in all parts below the sec- 

 tion. In this section, the only fibres that are divided are those passing 

 from one side of the cord to the other. This decussation is by fibres 

 prolonged from the cells of the posterior cornua, which cross in the 

 gray commissure, around the central canal. 



When one lateral half of the cord is divided in a living animal, sen- 

 sibility is impaired or lost on the opposite side of the body below the 

 section, but there is hyperesthesia on the side corresponding to the 

 section. This exaggeration of sensibility has not been satisfactorily 

 explained. 



Conduction through Synapses. Repeated reference has already 

 been made to the so-called synapses that connect nerve-fibres with nerve- 

 cells. The cell-bodies, as has been seen, present each an axis-cylin- 

 der prolongation, the neurite, and branching, or arborizing processes, 

 called dendrites. In the theory of the transmission of nerve-impulses 

 or of centripetal currents through synapses, it is assumed that the con- 

 ducting neurite, when it reaches a cell-body, branches and arborizes 

 around the cell, its branchings interlacing with branching dendrites, but 

 without direct connection of filaments. This is called a synapse from 

 a-waiTTa), to join together. Following out an impulse from a motor area 

 in the cerebral cortex to a muscle, it evidently must pass through several 

 collections of cells, notably in the corpus striatum, the pons and the 

 multipolar cells on the opposite side of the cord. Each synapse has 

 been compared to a relay-station ; and the passage of the impulse, which 

 is taken up by the cells and conveyed onward by its neurite, involves a 

 delay which, in the frog, is about 0.002 of a second. In afferent conduc- 

 tion the mechanism of transmission of impressions is nearly the same. 

 In reflex action involving the cord only, arborizing filaments from the 

 afferent nerve-roots form synapses with the dendrites of the motor cells 

 and impulses are sent from these cells to muscles. It is thought, also, 

 that synapses exist in the ganglia of the sympathetic system in the 

 motor paths of conduction to non-striated muscles. The same mechan- 



