Secondary degenerations occur in the brain and spinal cord as a result of interruption of nerve fibres, and they are to be understood on similar principles to the degeneration of peripheral nerves from division of their fibres. The interruption to the nerve fibres may be in the brain, in the cord itself, or in the posterior nerve roots. Those occurring in the brain are mostly the result of haemorrhages, softenings, or other coarse lesions. In the cord itself interruption is the result of inflammation, of direct injurfes, or of lesions which press on the cord. Acute curvature of the spine, whether the result of injury, as when there occurs a fracture-dislocation of the vertebrae, or of disease, as in tuberculosis of the vertebrae, is a somewhat frequent cause. Pressure on the cord is produced in various ways. As the cord is within a rigid bony canal, any increase in the contents of the canal is apt to cause pressure on the cord. Thus haemorrhage into the spinal canal, such as results from an aneurysm bursting into the canal, may cause it. Again tuberculosis of the vertebrae, if it extends to the membranes of the cord, leads to great thickening of them, a pulpy granulation tissue being produced like that in tuberculosis of joints. The pressure of this new-formed tissue has, in several cases observed by the author, led to interruption to the cord. Again, tumours of the membranes or of the cord may have a similar result. Syphilitic lesions in some cases press on the cord and lead to interruption. Lastly, inflammations in the cord itself, by the pressure of the exudation or, in chronic inflammation, of the new-formed connective tissue may interrupt the nerve fibres.
We have already seen that when a nerve stem in an animal or in man is divided, the peripheral portion of the nerve degenerates. The most prominent changes are in the medullary sheath which coagulates, then breaks up, and is finally absorbed. The degeneration occurs from the point of section towards the periphery, and we have seen that the explanation of this seems to be that the nerve fibres are cut off from the nerve cells of which the axis cylinders are prolongations. In the central nervous system, when the fibres are interrupted, the degeneration is of the same character as in peripheral nerves, and results in the destruction of the medullary sheath.
The opaque dead white colour of the white or fibrous nerve substance is due to the medullary sheath, which is a highly refracting fatty substance. If this medullary sheath be lost, then the white nerve substance becomes grey, and so we speak of Grey degeneration in all cases where the medullary sheath is lost, whether from the cause we are considering or not. A tract of white nervous tissue affected with this degeneration will be like a tract of grey substance.
Fig. 307. - Sclerosis or grey degeneration of cord. To the right is normal white substance. To the left the degenerated tissue shows a granular basis-substance in which a few nerve fibres are still visible. From a case of descending degeneration. X 80.
When a degenerated area is examined under the microscope the most marked change is the great reduction in the number of nerve fibres, as shown in Fig. 307. The connective substance is greatly increased, and the nerve fibres appear only at intervals, a few surviving in the midst of the general destruction. There is sometimes also considerable shrinking of the degenerated area, but this is much more manifest in the medulla oblongata and parts above it than in the cord-In addition to simple degeneration there is sometimes a new-formation of connective-tissue, which some regard as inflammatory in character. The inflammatory character is-little marked in secondary degeneration, but there are some forms of grey degeneration in which the inflammatory new-formation is-perhaps the primary factor, and the degeneration of the nerve fibres the result of it. Whether there be new-formed connective tissue or not, and whether the process be inflammatory or not, the absence of the soft medullary sheath causes a hardening of the tissue, and so the term Sclerosis when applied to the white substance is nearly equivalent to grey degeneration.
It has been said above that the degeneration occurs in the fibres which are cut off from their cells, and it seems that in the cord as a rule the cells are at the lower termination of the afferent fibres and at the upper termination of the efferent. Hence the degeneration mostly follows the direction in which the nerve conducts, passing upwards from the seat of lesion in the case of centripetal fibres and downwards in the case of centrifugal. So we speak of ascending and descending degeneration.
The degeneration takes some time to develop. According to experiments in dogs it begins fourteen days after the infliction of an injury, but it is many weeks before the appearances are fully established.
Fig. 30S. - Descending degeneration in medulla oblongata and cord. a, Medulla at fourth ventricle, b, at decussation, c, d, e, f, cord in upper and lower cervical, dorsal, and lumbar regions.