LESIONS OF THE SPINAL MEDULLA 



45 



wards, it increases in size owing to the accession of new 

 fibres. In the medulla oblongata and the pons, it receives 

 fibres from the sensory nuclei of the cerebral nerves of the 

 opposite side. 



Lesions in the medulla oblongata may affect the spino- 

 thalamic tract together with one of the sensory cerebral nuclei, 

 the fifth being most frequently involved (p. 61). The result- 

 ing condition is akin to crossed paralysis (p. 39) and is termed 

 alternate hemianasthesia, because the anaesthesia affects the 



FIG. 26. Diagram to illustrate the course taken by Sensory Fibres 

 after entering the Spinal Medulla. 



A. Spino-thalamic tract (painful, thermal 



and tactile sensations). 



B. Posterior funiculus of spinal medulla 



(muscle and joint sense, and a few 

 tactile fibres). 



C. Anterior nerve-root. 

 \D. Posterior nerve-root. 



E. Anterior ramus (primary division). 



F. Posterior ramus. 



K. Typical spinal nerve. 



limbs of the opposite side and the trigeminal nerve on the 

 same side as the lesion. 



In a Hemi-lesion of the Spinal Medulla in the mid-thoracic 

 region, the lower limb of the same side is completely paralysed 

 owing to the interruption of the lateral pyramidal tract, and, 

 since it is the upper neurone which is affected, the paralysed 

 muscles are spastic. In addition, there is loss of muscular 

 and joint sense in the paralysed limb, since the fibres which 

 convey these varieties of sensibility do not decussate until 

 they reach the medulla oblongata. On the other hand, the 



