THE NERVOUS SYSTEM. 813 



frequently the sclerosis is not wholly confined to the motor tracts, but 

 extends into the neighboring anterior and lateral tracts, especially in 

 the cervical region. The degeneration in these tracts has been traced 

 through the medulla, pons, and cms to the cortex, where changes have 

 been observed in the large pyramidal cells. In the gray matter of the 

 medulla are also found changes analogous to those in the anterior horns, 

 consisting in degeneration of the cells of the motor cranial-nerve nuclei 

 (hypoglossal, spinal accessory, pneuniogastric, glosso-pharyngeal, and 

 facial). The atrophy of the muscles innervated by these nerves gives 

 the clinical picture of glosso-labio-laryngeal paralysis or progressive 

 bulbar paralysis. The medullary lesion may occur without any corre- 

 sponding lesion of the cord, thus giving rise to a bulbar paralysis with- 

 out any spinal symptoms. The seat of the initial lesion, whether in the 

 cell bodies, in the cortex, or in their axones, is as yet undetermined. 

 The fact that disturbances of nutrition acting primarily upon the cell 

 body are frequently most evidenced by changes in parts of the neurone 

 farthest from the trophic centre has already been mentioned. The mus- 

 cle changes are those of a progressive muscular atrophy. 



DEGENERATION OF THE PERIPHERAL SENSORY NEURONES. (Tabes 

 Dorsal is Posterior Spinal Sclerosis Locomotor Ataxia. ) 



The essential lesion of tabes is a primary progressive degeneration of 

 the peripheral sensory neurone system. As its older title indicates, the 

 clinical picture of locomotor ataxia had been attributed to a primary 

 sclerosis of the posterior columns. This was not because in these col- 

 umns was the only lesion, but because, owing to the close packing to- 

 gether here of the sensory axones, this lesion was the most conspicuous. 

 The distribution of the lesion is coextensive with the peripheral seusorj- 

 neurone system (page 808). Any part of the system may be affected or 

 the lesion may involve practically the whole system. Again, the entire 

 neurone may be affected, or only part of the neurone, or one part more 

 than other parts. In general it may be said that it is ihe central process 

 of the neurone, which enters the cord in the posterior nerve roots, that 

 shows the earliest and most extensive changes. 



It is convenient to classify the lesions of tabes according to the anat- 

 omy of the peripheral sensory neurone system, as follows: 



1. The Peripheral Processes. () The sensory nerve endings. Changes 

 of a degenerative character have been described in the sensory nerve 

 endings of the skin, joints, and muscle. Especially well marked are 

 those of the (sensory) muscle spindles which are almost constantly 

 present. 



(6) The peripheral nerves. Degenerations of the peripheral nerves 

 are common. Like similar conditions due to toxic agents, they are usu- 

 ally referred to as a neuritis. The changes in both axones and medullary 

 sheaths are, however, degenerative in character and similar to those of 



