CHAPTEE XLIX 



FUNCTIONS OF THE SPINAL CORD 



THE functions of the spinal cord fall into two categories : functions 

 of the grey matter, which consist in the conversion of afferent into 

 efferent impulses (reflex action) ; and functions of the white matter, 

 which are those of conduction. 



The Cord as an Organ of Conduction. 



The fibres of the spinal cord consist of three main groups ; these 

 are (1) the association tracts, which connect together different 

 segments of the cord and thus bring about coordination of the 

 impulses, which leave it at various levels, in relationship to the 

 impulses which enter the cord either below or above any particular 

 region ; (2) the efferent projection fibres, which connect the cord with 

 the different parts of the brain above it. The main motor path, the 

 pyramidal tract, comes into this category ; and after our full descrip- 

 tion of its course, we need not do more here than remind the reader 

 that it originates from the giant pyramids of the motor area of the 

 cortex, and that its fibres cross to the opposite side of the spinal 

 cord, the principal decussation occurring at the lowest level of the 

 bulb ; from the grey matter in which it terminates in the cord, the 

 impulse is continued onwards, until in the end it reaches the muscles 

 via the fibres which leave the cord in the anterior nerve-roots ; (3) 

 the afferent projection system of fibres; these primarily enter the 

 cord by the posterior spinal nerve-roots. The impulses which these 

 convey enter (a) the spinal grey matter, (6) the cerebellum, and (c) 

 the cerebrum, the seat of consciousness. 



It is these sensory tracts which are the most complex, not only 

 on account of the cell-stations in their course, but also on account of 

 the difficulty of determining in animals the different kinds of sensa- 

 tions which are present in health, or which may be absent after 

 injury to various tracts. It is, however, certain that ultimately, so 

 far as the cerebrum is concerned, crossing occurs somewhere, so that 



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