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 fibers, numbering more than 

 half a million, 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 otherwise 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 or motor 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 

 throughout the gray matter, and according to the side on which the 

 axon enters into a tract they may be divided into three subgroups: 



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