With respect to the reproduction of solutions of continuity in the nervous centres, what little is known must.be viewed as unfavourable to the supposition that perfect restoration of the lost prts takes place. If the bmin or spinal cord be wounded, union will take place; but it does not appear from Arnemann's observations nor from Flourens's that the uniting substance is true nervous matter. Further researches are much needed upon this interesting subject.
Abnormal anatomy of the spinal cord and its inembranes.—The membranes of the spinal cord are liable to those morbid changes which com monly affect the tissues of the same kind oc curring in other parts of the body.
Inflarnmatory affections of the dura mater are exceedingly rare, and occur chiefly in connexion with wounds or injuries of the spine, or in extension of disease from the bones. Occa sionally but very rarely we find osseous or cartilaginous deposits upon it, which are most obvious on its arachnoid surface. Blood is sometimes, but not frequently, effused externally to it, and effusions of serous fluid are still more rare. Such effusions, from the usually depen dent position of the spinal canal, and from the large venous plexus which exists around the dura mater, are very likely to be pseudo-morbid, resulting from the gravitation of the fluids after death. Cancerous or fungoid tumours may originate from the dura mater, or may arise externally and grow to it afterwards. Tuber cles may form between the dura mater and its arachnoid lining.
When a deficiency of more or less of the posterior osseous wall of the spinal canal occurs, we find a corresponding dilatation of the dura mater and arachnoid sac, which, being filled with water, causes an external tumour, consti tuting what has been c,alled Hydrorachis, the consequence of spina bifida. These tumours are altogether dependent on the congenital imperfection of the bones of the spine, and whatever peculiar disposition of the spinal cord or its nerves may be found within them is due to an arrest of or a disturbance in the process of developement of those parts. The details of the anatomy of these tumours will be found in the article Sp' NE.
The arachnoid.—The spinal arachnoid exhi bits marks of the inflammatory process more fre quently titan thed ura mater. But in neither mem brane does this state of disease occur often, ex cept as a complication of injury or of a morbid stateof other parts,either of the vertebral column or of the spinal cord itself. The signs of an
inflamed state of this membrane are lymph effused on its free surface, recent, or indurated causing more or less thickening. Adhesions be tween corresponding parts of the two arach noid layers are also a good indication of a previously existing inflammation. But care must be taken not to mistake the adhesion, which is often found between points of these membranes, for inflammatory adhesion. The former occurs in minute points, and is probably a result of drying of the membranes at the points of contact ; the latter is always ac companied by the formation of new matter which forms the medium of union between the layets.
Cartilaginous spots are by no means unfre quently found on the arachnoid membrane, chiefly in connection with its viscerallayer. They are generally small detached laminve thoroughly incorporated with the membran e,rarely exceeding in size the flat surface of a split pea, more fre quently much smaller. They generally occur on the posterior surface of the arachnoid. I have seen such deposits in cases where there were no previous symptoms to denote any affection of the central nervous system, and I am disposed to believe that such deposits, separated as they must be from the surface of the cord, are not likely to occasion much if any irritation to that organ, They are found mostly in the dorsal and lumbar regions. Sometimes spots of bone, of similar size and shape, are found scattered over the membrane, The pia mater.—The spinal pia mater being the seat of the vascular ramifications which con tribute to the nutrition of the cord, is also sub ject to congestions often depending on causes quite extraneous to the spinal canal or cord. Thus the congestions which are produced in animals drowned, or asphyxiated in any other way, exist in the vessels of this membrane. The most frequent cause of congestion in these vessels, however, is the position of the corpse after death. After deaths, preceded by violent convulsions, there is always congestion of the vessels of the pia mater. But this congestion must be regarded as a consequence, not as a cause of the convulsions. The holding of the breath, which accompanies continued convul sions, gives rise to a very general congestion of the venous system.