HISTOLOGY. 



from other parts of the cord. Toward the tip of the dorsal column there 

 is a macroscopic, apparently gelatinous mass called the gelatinous substance 

 (substantia gelatinosa); and dorsal to this there occur successively the 

 spongy zone, and the terminal zone (zona spongiosa and zona terminalis). 

 The latter consists chiefly of nerve fibers running lengthwise of the cord. 

 The dorsal median septum, generally described as formed of compressed 

 strands of neuroglia, is well marked; it resembles the ventral median 

 fissure since the walls of the latter have been brought so close together. 



Structure of the cord. From the preceding account of the develop- 

 ment and topography of the cord, it is evident that there are three layers 

 to be examined, the white layer, the gray layer, and the ependyma; these 



may be considered in 



Blood vessel. A. , tUITl. 



The white sub- 

 / stance [matter] con- 

 sists of a syncytial 

 framework of neurog- 

 lia through which 

 pass blood vessels, 

 and nerve fibers 

 mostly medullated. 

 The myelin sheaths 

 of the latter produce 

 the very white macro- 

 scopic appearance of 



Short rayed cells. Long rayed cells. this layer when freshly 



FIG. 142. NEUROGLIA CELLS FROM THE BRAIN OF AN ADULT MAN. ,-,-,, r 



GOLGI METHOD, x 280. cut. The nature 01 



the neuroglia syn- 



cytium is seen in the longitudinal section, Fig. 141. Stiff fibrils 

 have developed in its exoplasm, and they are continuous from one 

 cell territory to another. As the nerve fibers which occupy the neuroglia 

 meshes increase in number, and in size by becoming medullated, the 

 neuroglia nuclei surrounded by protoplasm are compressed into stellate 

 forms (Fig. 144, A). In the Golgi preparations they appear as in Fig. 142, 

 and are described as long rayed, and short rayed or mossy cells. These 

 forms represent clumps of neuroglia fibers, sometimes clogged with 

 precipitate, in the center of which there may or may not be a nucleus. 

 Fig. 143 shows the appearance of the neuroglia riet in ordinary sections. 

 Over the outer surface of the cord it makes a dense feltwork, generally 

 free from nerves. It has been called the external limiting membrane. 

 Outside of it is a very vascular connective tissue layer, the pia mater. The 



