SYEINGOMYELIA. 225 



projections of the nerve roots. Skin reflexes vary in their activity 

 inversely with the degree of local cutaneous anaesthesia. 



A convenient means of testing the loss of tone in the muscles is 

 found by taking advantage of the fact that the hamstrings, when 

 possessing their normal tone are not of sufficient length to allow 

 the leg to be fully extended on the thigh at the same time that 

 the thigh is fully flexed on the abdomen. This position can 

 often, however, be assumed in tabes, and certain cases of 

 peripheral neuritis in which muscle tone is lost. 



The perforating ulcer of the foot usually originates at the site 

 of a corn, and is obviously due to pressure and suppuration in 

 anaesthetic tissues. 



The trophic joint and bone lesions of tabes are usually 

 attributed to the lesion of special trophic fibres, which are 

 supposed to follow the paths taken by the fibres for pain and 

 temperature rather than those of the other afferent tracts. 



The Argyll Eobertson pupil has been discussed in the section 

 on ophthalmoplegia. 



The intraocular portion of the optic nerve is accessible to 

 direct examination, and the occurrence of primary optic atrophy 

 may afford valuable evidence in doubtful cases of tabes. Of 

 the other cranial nerves we can only judge by function. The 

 ascending root of the fifth nerve, which corresponds to the pos- 

 terior roots of the cord, is sometimes involved, and so bilateral 

 facial neuralgia exceptionally forms a symptom of tabes, corre- 

 sponding to the lightning pains and to the sciatica of the 

 lumbo-sacral form of the disease. 



SYRINGOMYELIA. 



Syringomyelia is characterised by the presence of a dilated 

 central canal or of other cavities within the spinal cord, together 

 with more or less proliferation of the embryonic tissue in their 

 neighbourhood. The lesions are practically confined to the 

 posterior half of the cord, and usually limited to the posterior 

 columns, which are formed last during the process of develop- 

 ment. The disease is a congenital defect, the primitive canal 



C.A.A. 15 



