THE NERVOUS SYSTEM. 811 



of the cord which interrupts the tract of Gowers or the direct cerebellar 

 tract is followed by degeneration of the fibres of these tracts above the 

 lesion (Fig. 530). Immediately above the lesion there is complete de- 

 generation of the tracts. As we pass upward new imdegenerated fibres 

 from the spinal ganglia above the lesion enter the column of Burdach, 

 so that there appears in the posterior columns a constantly increasing 

 number of normal fibres. Also in the column of Gowers imdegenerated 

 fibres appear as one passes upward from the lesion. Some of these fibres 

 probably have their origin in the gray matter of the cord, and the num- 

 ber of normal fibres is in proportion to the number of these cells be- 

 tween the lesion and the point at which the section is taken. Descending 

 fibres in these columns have already been mentioned. They, of course, 

 do not degenerate. If the lesion be above Clarke's columns the degen- 

 eration in the cerebellar tracts remains complete ; if not, the number of 

 normal fibres is in proportion to the number of Clarke's column cells 

 between the lesion and the point of section. The ascending fibres of the 

 fundamental columns also degenerate, but are so short that they cannot 

 usually be traced beyond the area of direct action of the traumatism. 



Primary Degenerations. 

 DEGENERATION OF THE PERIPHERAL MOTOR NEURONES. 



Progressive spinal muscular atrophy is the clinical designation of 

 a disease, the underlying lesion of which is a progressive atrophy or 

 degeneration of the lower motor neurones. There is a degeneration of 

 the large motor cells of the anterior horns and of their processes. This 

 degeneration goes on to complete destruction of some neurones. With 

 the loss of nerve tissue proper there is a compensatory growth of neu- 

 roglia. In a well-advanced case section of the cord shows a marked 

 diminution in the number of anterior horn cells and an atrophic condition 

 of the horn itself and of the anterior roots. The lesion usually begins in 

 the cervical region. More rarely it starts in the lumbar cord. The de- 

 generation may extend to the motor cranial nerve nuclei, giving the 

 picture of a progressive bulbar paralysis. The medullary nuclei most 

 commonly involved are the hypoglossal and the spinal accessory. Less 

 often the degeneration affects the cells of origin of the fifth and seventh. 

 Degeneration of the peripheral nerves has been described. The muscu- 

 lar lesions correspond to the lesions in the cord, the muscles of the hand 

 and arm being usually first affected. For the degenerative changes in 

 the muscles see page 754. This lesion is sometimes described as a 

 chronic antero-polioniyelitis. Both its clinical history and its pathology, 

 however, indicate the degeneration as the initial lesion and the ueuroglia 

 increase as secondary a replacement hyperplasia. 



