ACCESSORY ELEMENTS IN THE NERVE-CENTEES. 55 



covering, some distance removed from the cell itself so as to 

 oe nearly twice the diameter of the cell, which is continuous 

 with the sheath of the dark-bordered fibres. 1 This mem- 

 brane is always nucleated, and Kolliker has lately shown 

 hat it is not homogeneous, as was at one time supposed, but 

 is composed of a layer of very delicate epithelium. 8 The 

 physiological significance of this covering is not apparent. 



In the gray matter of the nerve-centres, there is a, finely- 

 granular substance between the cells, which closely resem- 

 bles the granular contents of the cells themselves. In addi- 

 tion to this granular matter, Robin has described new ana- 

 tomical elements which he has called myelocytes. These 

 are found in the cerebro-spinal centres, forming a layer near 

 the boundary of the white substance, and are particularly 

 abundant in the cerebellum. They exist in the form of free 

 nuclei and nucleated cells, the free nuclei being by far 

 the more numerous. The nuclei are rounded or ovoid, with 

 strongly-accentuated borders, are unaffected by acetic acid, 

 finely granular, and generally -without nucleoli. The cells 

 are rounded or slightly polyhedric, pale, clear, or very slightly 

 granular, and contain bodies similar to the free nuclei. The 

 free nuclei are from S J- OQ to -^m of an inch in diameter, and 

 the cells measure from -g-gVo to 2 0*0 o ? and sometimes 14 1 00 of 

 an inch.* These elements also exist in the second layer of 

 the retina. 



There has been a great deal of discussion with regard to 

 the presence or absence of connective-tissue elements in the 

 cerebro-spinal centres. In the other ganglia, there has never 

 been any doubt with regard to the presence of connective 

 tissue in greater or less amount, and in the cerebro-spinal 

 centres there can hardly be any question of the existence of 

 an exceedingly delicate stroma, chiefly in the form of stel- 



1 SCHULTZE, in STRICKER, Manual of Human and Comparative Histology, 

 London, 1870, vol. i., p. 173, el seq. 



2 KOLLIKER, Elements d'histologie humaine, Paris, 1868, p. 329. 



8 LITTRE ET ROBIN, Diclionnaire de medecine, Paris, 1865, Article, Myelocytts. 



