46 THE NERVOUS SYSTEM 



conduction of painful, thermal and tactile sensations from 

 the paralysed limb is not interfered with, since these fibres 

 cross the median plane as soon as they enter the spinal 

 medulla. The lower limb of the opposite side is not paralysed, 

 but, since the spino-thalamic tract is involved, it is rendered 

 completely anaesthetic to painful and thermal sensations and 

 partially anaesthetic to tactile stimuli. Muscle sense, however, 

 is unimpaired on the unparalysed side. 



Owing to the difference in length between the spinal 

 medulla and the vertebral canal (p. 40), the upper limit of 

 the anaesthetic area is considerably lower than the actual 

 level of the lesion. At the level of the upper limit of 

 anaesthesia, but on the paralysed side of the body, there is 

 a narrow zone of complete anaesthesia. This condition is due 

 to injury of the sensory fibres as they enter the spinal medulla, 

 and it affects the same side as the lesion, since the fibres 

 are implicated before they cross the middle line. 



The crossed motor and sensory paralysis which results from 

 a hemi-lesion of the spinal medulla is known as Brown- 

 Sequard Paralysis. 



In Acute Anterior Poliomyelitis the lesion is confined to 

 the anterior column of the grey matter of the spinal medulla 

 and it consequently produces a purely motor paralysis. 

 Further, it is usually restricted either to the cervical or the 

 lumbar enlargement of the spinal medulla, but, in severe cases, 

 it may be more widespread. The areas of grey matter affected 

 by the lesion are identical with the areas supplied by the 

 anterior spinal arteries, and it is believed that the organism 

 which produces the disease reaches its destination by the 

 blood-stream. 



Many of the muscles which are paralysed in the acute stage 

 completely recover at a later period. The residual paralysis 

 is of the lower neurone type (p. 3). Consequently, the para- 

 lysed muscles are flaccid; they undergo atrophy and their 

 electrical reactions become altered. 



In Progressive Muscular Atrophy the lesion may commence 



