CHAPTER LXXII 

 THE SPINAL CORD 



THE nerve-fibres act as conductors connecting the receptor and 

 effector mechanisms. The conductors form the nerve trunks of tin- 

 body the cranial and the spinal nerves and the tracts within the 

 central nervous system. The conductors are the processes of tin- 

 nerve cells, or neurons ; these interlace and form synapses, through 

 which the nervous energy is transinitted from one neuron to another. 

 We have to consider the ingoing neuron, which connects the 

 sensory nerve-ending, or receptor, to the central system, and the 

 efferent neuron the final common path which connects the 

 central system to the effector organ. The axons. or conductors, 

 of these two sets of neurons form the mixed nerve trunks of the 

 body. The arrangement of each spinal nerve is as follows: On the 

 postero-lateral aspect of the spinal cord there enters the posterior 

 root, which has a ganglionic swelling upon it the posterior root 

 ganglion. From the antero-lateral aspect of the cord emerges the 

 anterior root. The two roots combine to form the spinal nerve. In 

 all the thoracic nerves, and some of the sacral, there are not only large 

 fibres, which pass to the body -wall structures, and are known as 

 somatic fibres, but also small fibres which supply the viscera and 

 the involuntary muscle of the body, and are known as splanchnic 

 fibres. The course of these latter fibres is dealt with when the 

 autonomic system is considered (see p. 748). 



The fibres of the posterior root form the great afferent system. 

 the fibres of the anterior root the great efferent system. Section of 

 a posterior root leads, therefore, to a cutting-off of the impulses which 

 come both from the extero-ceptive and proprio-ceptive mechanisms 

 that is, the sense organs which receive impulses from the outside 

 world, and those which initiate the impulses which arise in the inner 

 world of the body itself. In the case of the spinal nerve, the former 

 impulses include those of touch, temperature, pain, and the latter 

 those of the kinaesthetic sense. Section of an anterior root cau> - 

 a paralysis of the muscles and any other effector organ supplied by 

 the nerve. Wallerian degeneration affects the ax cms. which are cut 

 off from the cell bodies of the neurons. 



The cells which give origin to the fibres of the posterior root are 

 situated in the posterior root ganglion; those of the anterior root 

 fibres in the anterior horn cells of the spinal cord. As a consequence 

 of section of an anterior root, the nerve-fibre degenerates towards 



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