THE SPINAL CORD 25 



up the cord to level A, where it reaches and excites the (5) motor cells m and (6) m. It 

 also passes upward to the basal ganglia, giving rise there to automatic acts, like the cry 

 of pain (7); or. to the brain cortex (8); producing there a conscious perception of the 

 sensation. The latter are not represented in the diagram. 



The greater the intensity of the sensation, the more marked are its effects in motion. A 

 slight sensation may cause a slight movement on the same side limited to the muscles moved 

 by the cells lying at the level of the cord which the sensory impulse reaches. A very severe 

 sensation may throw into activity the motor cells of both sides at all levels of the spinal cord 

 above and below the level at which it enters. The simple knee jerk is an illustration of the 

 former, the intense rigidity of all the muscles caused by the pain of a lumbago is an illustra- 

 tion of the latter. And all degrees of reflex action between these extremes are possible. 

 Any break in the course of these impulses by a defect in the reflex arc will cause a sus- 

 pension of reflex action. Hence neuritis by destroying the nerve fibres in the nerve trunk ; 

 posterior sclerosis by destroying the nerve fibres in the root zone of the column of Burdach ; 

 myelitis by destroying the nerve fibres in their passage through the gray matter; and anterior 

 poliomyelitis by destroying the motor cells in the anterior horn, cause a cessation of reflex 

 activity. 



Plate VII. shows a longitudinal section through the spinal cord of a human embryo of 

 eight months, and demonstrates not only the appearance of the large cells of the median 

 region with their dendrites, but also the fine plexus of interlacing nerve fibres, dendrites, and 

 neuraxons in the gray matter, seen on the left; and the longitudinal fibres of a white column, 

 seen on the right. From the fibres of this white column, several collaterals can be seen to 

 come off at right angles and to pass into the gray matter. Around the middle cell in the 

 plate, a few fine terminal filaments of such collaterals can be distinguished with free ends. 



