As we have just seen, this lesion affects centrifugal or motor fibres, including the fibres of the anterior root-zone and the pyramidal tract. The former are short fibres, and when they are interrupted there is a descending degeneration extending only a short distance downwards. But the fibres of the pyramidal tract are continuous from the brain downwards to the extremity of the cord, and wherever interrupted they show degeneration in all parts situated below the lesion.

The degeneration of the pyramidal tract is most frequently brought about by a Lesion of the brain, and in that case it exists in the parts of the tract above as well as in those below the medulla oblongata. We have here to consider it in the medulla and cord. In the medulla oblongata it occupies the anterior pyramid (see Fig. 308, a), where it frequentty produces great shrinking. At the lower part of the medulla (Fig. 308, b) the degenerated fibres decussate and the degeneration takes up its position at first at the side of the central canal (see Fig. 308), afterwards, in the cord, passing into the lateral columns in the regular position of the pyramidal tract (Fig. 308, c, d, e, f, and Figs. 309 and 310). In some cases it entirely decussates, but the column of Turck in the anterior white column is also affected in most cases. In its whole course the lesion diminishes from above downwards, but is traceable down to the lumbar region. The reason for this is that the pyramidal tract gradually diminishes in bulk from above downwards on account of its fibres leaving it from time to time to come into relation with the branching processes of the large motor cells in the anterior horns. It may be noted also that, unlike what has been described as occurring on section or interruption of a spinal nerve, when the fibres of this tract suffer interruption or injury, degeneration occurs simultaneously in the whole length of the fibre distal to the point of interruption, and is only a descending degeneration in the sense that it occurs below the point of injury.

Fig. 309. - Descending degeneration. Cervical region. The pyramidal tract on the right side is pale.

Fig. 310. - Descending degeneration. Lumbar region. The affected pyramidal tract small and reaching the surface.

The pyramidal tract may be interrupted in any part of its course, and it may be so in the cord. If the entire cord be divided there is, immediately beneath the point of section, a degeneration of the anterior root-zones which is continued but a short distance. There is also a degeneration of the pyramidal tract on both sides continued downwards the whole length of the cord. The interruption either in the cord or brain may be incomplete, and in that case the degenerated fibres will be fewer in number and the area less distinctive in appearance.

Perhaps it should be mentioned that now and again in cases of unilateral cerebral lesion giving rise to descending degeneration in the pyramidal tracts, the degeneration is double, involving the tracts of both sides. The explanation of such anomalous cases must be found in irregularity in the distribution of the pyramidal fibres, but in all cases at least a few fibres may be found degenerated in the situation of the pyramidal tract of the same side as the lesion or idio-lateral.

Effect On Function Of Descending Sclerosis

We have already seen that the pyramidal fibres end in the cord at successive levels, passing into the ganglionic centres. When the fibres are degenerated these centres are cut off from the higher centres and left more to themselves. They are still connected with the muscles, which retain their contractile power. Voluntary motion is lost, but certain involuntary muscular phenomena may be even exaggerated.

Late rigidity, occurring in hemiplegia, or in paraplegia from injury to the cord, comes on a considerable time after the onset of the paralysis, and may be regarded as coinciding in time with the full development of the sclerosis. There is here a more or less permanent contraction of some muscles with absence of contraction in others, producing sometimes fixation of the members of the body in special positions so as to have the appearance of deformity. This fixed condition, due to the muscular spasm, is often called Contracture. It implies a continuous impulse to the contracted muscles originating in the cord or elsewhere. Charcot suggests that the inflammatory process which he supposes to be involved in the sclerosis may irritate the fibres passing to the anterior cornua and so result in the stimulation of the muscular centres there. But this view* can hardly be accepted; there may be almost no signs of inflammation in the affected part and yet marked rigidity. Besides, it is difficult to understand how irritation of degenerated and virtually lost fibres should cause stimulation of the ganglion cells. A more probable explanation is suggested by Hitzig. Taking as an example the case of the arm in hemiplegia, it appears that the contraction occurs most readily in those muscles which are in a position to contract most easily. The hemiplegic generally lies in bed with his forearm across his chest, and even when walking about he supports it across his chest, and it is the biceps which becomes rigid. Then it has been shown that when the fingers are released from the action of the muscles and left to assume the position to which the bones and ligaments best accommodate themselves, they assume a semi-flexed position, such as we see in the dead body. It is obvious that the flexor muscles will most easily contract under these circumstances, and it is they which get rigid in hemiplegics. Take along with this the fact which Volkmann points out, that muscles ean actively contract but cannot actively relax, and it is seen that any slight impulse is apt to be cumulative when there is no action of Antagonistic muscles. The centres of the anterior cornua are still in connection with the muscles, and although cut off from the upper brain .are still exposed to irregular and, as it were, accidental stimulation. There are reflex stimuli, and there are stimuli from above conveyed in a, roundabout way through the still open communications in the cord. A multitude of slight stimulations will reach the centres and as the result of " summation of stimuli" feeble impulses will be conveyed to the muscles. Those which are stretched will not contract, but those which are so placed as to contract easily will do so, at first feebly but with cumulative force. This view is supported by the fact that, in the early .stages of late rigidity, there is often considerable relaxation of the muscles after prolonged rest, as in sleep, so that a limb which was rigid at night is found in the morning soft and movable. The ganglion cells in sleep are protected from external stimulation and they cease acting. Pierre Marie, on the other hand, is inclined to the opinion that the rigidity may be the result of the withdrawal of the inhibitory function of the pyramidal tracts upon the centres in the cord and hence their ill-timed and unceasing action.

The exaggerated Muscle reflex which is exemplified in the familiar clinical demonstration of the Knee-jerk and ankle-clonus, may be regarded as due to the isolation of the muscular centres in the cord. When the control of higher centres is removed, lower centres usually act more readily. The skin reflex is frequently decreased, but the muscle centres seem to be more powerfully acted on by stimuli coming from the muscles.