Spinal Reflexes 219 



s, a nurse ; rpe$o>, nourish) filaments from these cells run with the 

 motor nerves. Inflammation of these cornua is called anterior polio- 

 myelitis (TTO\IOS, grey; pveXos, marrow); it sometimes follows in the wake 

 of diphtheria, or, suddenly and obscurely occurring in a healthy child, 

 causes infantile paralysis. This condition may also occur in the 

 adult, however, as well as in the infant. When the cells are destroyed, 

 not only are the muscles flabby and useless, but, together with the 

 bones, their nutrition is affected and their development ceases, and 

 they no longer respond to Faradism. The excito-motory circuit being 

 broken, reflexes are lost, but, the posterior track of the cord being 

 uninterfered with, sensation is not impaired. 



It does not follow that the paralysis after polio-myelitis will be 

 permanent. The cells which are placed in the centre of the storm- 

 region are often completely wrecked, while many of the outlying cells 

 receive only a passing shock. Sometimes after such a storm, in the 

 cervical enlargement, for instance, all the muscles of an upper extremity 

 are paralysed ; but the power of movement may return again in all, 

 with the exception, perhaps, of one small group of muscles. Some- 

 times only a single muscle is left permanently paralysed the deltoid, 

 for instance. 



In the early days of infantile paralysis there is often a tenderness 

 or a hyperaesthesia of the skin of the affected limb. The explanation 

 of this is that the storm-wave happened also to disturb the posterior 

 cornu, with which the posterior roots of the nerves are associated. 



Progressive muscular atrophy differs from the paralysis just 

 considered in that it is the result of a slow degenerative change not a 

 rapid inflammatory one in the ganglionic cells. But, the motor and 

 trophic cells only being diseased, there is no loss of sensation in the 

 affected parts, though the muscles affected grow steadily smaller and 

 weaker. 



Reflex action in cord. The sensory impulse conveyed through 

 a spinal nerve passes by the posterior root into the grey crescent, and 

 then, traversing the large bi-polar cells of the anterior cornu, is con- 

 verted into a motor one, which is ' reflected ' by the anterior root of the 

 spinal nerve and causes certain muscles to ' act.' 



Thus, if the sole of the foot be tickled during sleep when the brain 

 has handed over general control to the reflex centres the impulse is 

 transmitted through the crescent in the lumbar enlargement to the 

 motor filaments, certain muscles contract, and the foot is drawn away. 

 But if the man be awake the sensory impulse passes at once across 

 the grey commissure and up the opposite half of the cord to the brain, 

 where it is duly appreciated, and whence it is reflected as a motor im- 

 pulse by the pyramidal tracts, and then out by the anterior root to the 

 muscles. Or ' we can, if we wish, execute voluntarily a movement of 

 the leg quite the same as the reflex act. Moreover, we can exercise 

 some voluntary control over the reflex action and prevent the start of 



