THE NERVOUS SYSTEM. 835 



of gliomatous or glio-sarcomatous tissue in the vicinity of the central canal, 

 and its subsequent partial disintegration with the formation of one or more 



FIG. 545. SYRINGOMYELIA. 

 An irregular cavity in the gray matter of the spinal cord, lined by a thick layer of gliomatous tissue. 



cavities within the substance of the cord (Fig. 545). These cavities, 

 which are filled with fluid, vary greatly in size, shape, and extent, and, 

 while usually situated in the central region of the cord, may involve the 

 anterior and posterior cornua and invade the posterior columns. There 

 may be two communicating cavities, and these may, but usually do not, 

 open into the central canal. The longitudinal extent of these cavities 

 varies greatly. The lower cervical and upper dorsal regions are most 

 frequently involved. The cavity is usually lined with tissue somewhat 

 denser than that which makes up the bulk of the tumor. The gliomatous 

 or glio-sarcomatous tissue which forms the basis of the lesion in syringo- 

 myelia probably originates from the layer of neuroglia which surrounds 

 or extends away from the central canal. 



Syriugomyelia is frequently mistaken for hydromyelia which is a 

 congenital malformation, and in which the longitudinal cavity in the 

 cord is at some period lined with epithelial cells. Syriugomyelia has 

 also been confused with hseinatomyelopore. 



There seems, furthermore, to be a class of lesions of the cord, usually 

 classed as Syriugomyelia, in which cavities of various forms coexist with 

 a tumor in the vicinity of the central canal. But these cavities do not 

 appear to be formed by a breaking down of the tumor tissue, but in some 

 other way as yet little understood. 



TUMORS OF NERVES. 



The tumors of the nerves may be divided into those consisting largely 

 of or containing new-formed nerve tissue true neuromata, and the so- 

 called false neuromata, which are for the most part fibromata or niyxo- 

 niata originating in the connective tissue of the nerve (see Neuroma). 

 Myxo-sarcomata are less common, and primary sarcomata rare. The 

 nerves may be secondarily involved in sarcomata or in carcinomata 



