CHAPTER VIII. 

 THE SPINAL CORD AS A PATH OF CONDUCTION. 



In addition to the varied and important functions performed 

 by the cord as a system of reflex centers controlling the activities 

 of numerous glands and visceral organs as well as the so-called 

 voluntary muscles, it is physiologically most important as a path- 

 way to and from the brain. All the innumerable fibers that 

 enter the cord through the posterior roots of the spinal nerves 

 bring in afferent impulses, which may be continued upward by 

 definite tracts that end eventually in the cortex of the cerebrum, 

 the cerebellum, or some other portion of the brain. On the other 

 hand, many of the efferent impulses originating reflexly or other- 

 wise in different parts of the brain are conducted downward into 

 the cord to emerge at one or another of the anterior roots of the 

 spinal nerves. The location and extent of these ascending and 

 descending paths form a part of the inner structure of the cord, 

 which is most important practically in medical diagnosis and which 

 has been the subject of a vast amount of experimental inquiry in 

 physiology, anatomy, pathology, and clinical medicine. In working 

 out this inner architecture the neuron conception has been of the 

 greatest value, and the results are usually presented in terms of these 

 interconnecting units. 



The Arrangement and Classification of the Nerve Cells 

 in the Gray Matter of the Cord. Nerve cells are scattered 

 throughout the gray matter of the cord, but are arranged more 

 or less distinctly in groups or, considering the longitudinal aspect 

 of the cord, in columns the character of which varies somewhat in 

 the different regions. From the standpoint of physiological anatomy 

 these cells may be grouped into four classes: (1) The anterior 

 root cells, clustered in the anterior horn of gray matter (1, Fig. 68). 

 The axons of these cells pass out of the cord almost at once to 

 form the anterior roots of the spinal nerves. (2) The tract cells, 

 so called because their axons instead of leaving the cord by the 

 spinal roots enter the white matter and, after passing upward or 

 downward, help to form the tracts into which this white matter may 

 be divided (2 and 3 of Fig. 68) . These tract cells are found through- 

 out the gray matter, and according to the side on which the axon 

 enters into a tract they may be divided into three subgroups: (a) 



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