The affections to be here considered are almost all congenital, at least in their origin. They are related to the central canal of the cord and its origin.

The central canal of the cord presents normally considerable variations. In its typical form it is a narrow slit lined with cylindrical epithelium. The tissue immediately around it is much more cellular than the rest of the cord, this being due to the fact that it consists almost alone of neuroglia, without nervous elements, in this respect comparing with the ependyma of the cerebral ventricles. From this typical condition the variations consist in different conditions of the canal itself and of the surrounding tissue. The canal may be widened either generally or in the transverse or median direction. On the other hand the canal may be obliterated and its position only indicated by the cellular area of neuroglia. These are all' within normal limits.

The term Hydrorrhachis corresponds with hydrocephalus, and expresses a dropsy of the cord, either of the central canal (H. interna) or of the meninges (H. externa). Dropsy of the central canal is also designated Hydromyelia and Syringomyelia. The former term is applied to cases where the dilatation is obviously congenital, while the latter is used where it is met with in the adult. Syringomyelia is however, probably always congenital in its origin, so that it may be said that it originates in a hydromyelia. These terms are applied to all cases of cavities or cysts in the cord filled with serous fluid whether they are demonstrably due to dilatation of the central canal or not.

It will be remembered that the central canal is formed by the arching backwards of the medullary plates (see p. 44), so that the closure is posterior. The central canal will, therefore, at first extend somewhat backwards, and it is by a gradual coalescence of the posterior portions .in the middle line that the canal takes its usual position. Dropsy occurring in foetal life is liable to cause a permanent enlargement backwards as shown in Fig. 340. The dilatation may vary greatly at different levels, so as to lead sometimes to a cystic appearance in the cord. In some cases cavities exist in the cord without obvious connection with the central canal, but as these are always in the posterior parts of the cord, they probably take origin in portions of the original canal, which in the process of coalescence have become isolated. These not infrequently extend into the posterior cornua, and may be double (Fig. 341) or single (Fig. 342).

An interesting peculiarity of these various conditions is that the serous cavities produced are surrounded, like the normal central canal, with neuroglia, which in some cases is in considerable excess. The excess may be so great as to amount almost to a tumour (glioma). The importance of this is emphasized by the occasional co-existence of syringomyelia with tumour of the cord, and even with multiple tumours (see Gowers). Gliomata of the cord mostly originate behind the central canal, even when there is no dilatation of the latter.

Hydromyelia. Enlarge ment of central canal backwards.

Fig. 340. - Hydromyelia. Enlarge ment of central canal backwards.

Syringomyelia. Cavities in each posterior cornu and in the middle line.

Fig. 341. - Syringomyelia. Cavities in each posterior cornu and in the middle line.

Syringomyelia. Cavity in one posterior cornu.

Fig. 342. - Syringomyelia. Cavity in one posterior cornu.

Hydromyelia and syringomyelia do not usually interfere with the function of the cord, but if the dilatation be excessive, or if there is a formation of tumour, there may be considerable damage.

Hydromyelia and syringomyelia, at least in the lesser degrees, may at times apparently exist without betraying their presence by any symptoms, but in many cases they are associated with a quite distinctive symptomatology. The condition has been recognized anatomically since the time of Brunner (1688), but such cavities were for long looked upon as mere peculiarities and without pathological significance, and it is only since Kahler and Schultze, some sixteen or seventeen years ago, showed that the condition was capable of recognition clinically that it assumed any importance.