VOLUNTARY MUSCLE. 755 



(Fig. 490). The walls of the blood vessels may also become thickened. 

 Hand-iii-hand with these interstitial alterations the atrophy of the muscle 

 fibres proceeds. These may simply grow narrower, retaining their stria- 

 tious; or they may split up into longitudinal fibrillse, or transversely 

 into discoid masses, and in this condition disappear. In other cases a 

 certain amount of fatty or hyaline degeneration may be present. These 

 degenerative and prol iterative changes do not, as a rule, occur uniformly 

 in the affected muscles, but some parts are affected earlier and more 

 markedly than others. The atrophied muscle may be replaced by fat. 



Progressive muscular atrophy is apt to commence in the small muscles 

 of the extremities, in many cases in the muscles of the ball of the thumb. 

 It may commence in the muscles of the shoulder, the arms, or the back. 



.;:'"*" |p 



Fir;. 495. PROGRESSIVE MUSCULAR ATROPHY (Soleus muscle, longitudinal section). 



a. Atrophied muscle fibre ; b, degenerated muscle fibre ; c, interstitial tissue ; d, clusters of proliferate 



muscle nuclei. 



It may have a continuous extension, or it may jump single muscles or 

 groups of muscles. Death may be induced by the affection of the 

 muscles of respiration or deglutition. 



The causes of this lesion are in many cases unknown, and there is 

 considerable lack of unanimity of opinion as to whether it is primarily a 

 disease of the muscles or of the nervous system. In a considerable pro- 

 portion of cases the muscle lesion is associated with atrophy of the gan- 

 glion cells in the anterior cornua of the spinal cord and the development 

 of connective tissue about them. In other cases these changes in the 

 cord may apparently be absent. 



It is sometimes accompanied by atrophy of the nerves which are dis- 

 tributed to the muscles, and atrophy of the anterior roots has been 

 described. 



It is probable that there are several varieties of progressive muscular 

 atrophy, which our present knowledge does not enable us clearly to dis- 

 tinguish. Muscular atrophy in some cases follows overstraining of groups 



