In its clinical aspects the main feature is a gradually progressive atrophy and consequent paralysis of the muscles. It very commonly begins in the muscles of the hand, but progresses from one muscle to another till the death of the patient. In its later stages it frequently becomes associated with a corresponding form of disease of the medulla oblongata, namely, bulbar paralysis, to be considered next.

In the muscles the change consists in what may be regarded as a chronic inflammation. The muscle-nuclei increase in number, and as the contractile substance diminishes, the sarcolemma may come to be filled with cells, the result of this proliferation of the muscle-nuclei. At the same time the interstitial connective tissue shows active changes, increase of nuclei, and new-formation of connective tissue. In the muscular substance various forms of degeneration have been observed, chiefly fatty and hyaline or simple atrophy. In any case the musculai fibres are lost by degrees and the connective tissue increased, but not sufficiently to make up the bulk of the lost muscular substance. Some times a fatty infiltration of the connective tissue occurs, so that adipose tissue comes to occupy the place of the muscle to a large extent. This change, if it occurs, is only local, and pure atrophy may exist side by side with atrophy with formation of adipose tissue. This formation of adipose tissue is mostly a late, and by no means a characteristic, occurrence in this disease.

In the Spinal cord the essential changes are in the anterior cornua. but in most cases the pyramidal tracts are also involved. The lesion in the anterior cornua consists in a gradual atrophy of the large ganglion cells, affecting the parts corresponding with the atrophied muscles. These cells may have en tirely disappeared or there may remain only small angular bodies representing them. There is at the same time an increase of the connective tissue, so that there is little change in the size or shape of the cornu.

According to Boss, the central grey column (the grey substance on either side of the central canal) is most affected, being traversed by large canals and fibrillated. In the annexed figure (Fig. 317) it is seen that the cells of the median group have also entirely disappeared, while the other groups of the anterior cornua are limited in size, the peripheral cells of the groups having disappeared, leaving only the more certain ones. It very commonly happens that, short of absolute destruction of the ganglion cells, they are greatly atrophied and pigmented (pigmentary atrophy).

Fig. 317. - Progressive muscular atrophy-section of cord in cervical region, from an advanced case. The central portion of grey substance is fibrillated; the median group of cells has disappeared, and the other groups are atrophied. (Ross).

The Pyramidal tracts, both those in the lateral and those in the anterior columns, are affected probably in all cases (Gowers), and the lesion has been traced up through medulla oblongata, pons, peduncles, internal capsule, and corona radiata to the motor convolutions of the brain, where -the large ganglion cells have been observed to be atrophied.

The Anterior nerve-roots are also atrophied. The nerve fibres may have almost entirely disappeared, and the nerves may be almost replaced by connective tissue. The degeneration exists throughout the peripheral distribution of these nerves, and the terminal fibres in the atrophied muscles are greatly altered.

Leyden has recorded a case of progressive muscular atrophy in which there was the peculiar condition of the cord designated Syringomyelia. (See further on).

Relation Of Lesion To Function

In this disease we have a degeneration or chronic inflammation affecting the motor system, chiefly in its lower parts; in other words, there is a degeneration of the spinal motor neuron. The disease centres in the ganglion cells of the anterior cornua, but it affects the nerve fibres arising from these, which are really prolongations of their processes, and in most cases also the pyramidal tracts above them. The degeneration of the pyramidal tracts is not to be regarded as in any sense secondary, but as part of the prolonged lesion of the motor system. Indeed the order in which the different parts are involved varies somewhat, and we may have the pyramidal tracts, at least in some parts of their course, affected in advance of the anterior cornua, although this is not common. We have already seen that spasm of the muscles is the characteristic result of degeneration of the pyramidal tracts, but this can only occur when the anterior cornua are unaffected. Hence in this disease rigidity of the muscles is not common, although in some cases it does occur locally, especially in the legs. Charcot has endeavoured to group this class of cases separately on the supposition that in them the lesion has begun in the pyramidal tracts, but it is doubtful whether this is a -utficient basis of distinction. In this latter class spastic paralysis precedes the wasting, and Charcot has applied the designation Amyotrophic lateral sclerosis to such cases.

But in most cases the anterior cornua are affected before or simultaneously with the pyramidal tracts, hence there will usually be no evidence during life of the affection of the latter.