NERVES 



NERVES. 



Nerve-cells and fibres from the auditory nerve. 1. Nerve- cell with the origin of a fibre, from the anastomosis 

 between the facial and auditory nerve in the meatus auditorius externus of the ox : a, cell-membrane ; b, contents ; 

 c, pigment; d, nucleus; e, prolongation of the sheath upon the nerve-tube; /, nerve-tube. 2. Two nerve-cells with 

 tubes from the auditory nerve of the ox : a, sheath with nuclei ; b, cell-membrane ; c, nucleus ; d, origin of tube, with 

 nucleated sheath. 3. Separate contents of a nerve-cell, with a nucleus and two nucleoli. 



Magnified 350 diameters. 



Fig. 519. 



Cells from the central gray nucleus of the human spinal 



marrow. 



Magnified 350 diameters. 



with nerve-cells, from which some of the 

 nerve-tuhes arise. The tubes and cells are 

 imbedded in or supported by a stroma of 



areolar tissue, sometimes homogeneous, at 

 others more or less distinctly fibrous, form- 

 ing an apparent sheath to the ganglia, and 

 ending in numerous septa ; rarely but 

 occasionally forming a distinct envelope to 

 the individual cells; sometimes it consists 

 of elongated, triangular, or spindle-shaped 

 nucleated cells, — in short, corresponding to 

 areolar tissue in various stages of develop- 

 ment. 



The nerves are developed from the ele- 

 mentary embryonic cells, which at first ap- 

 pear rounded, or slightly elongated, and 

 somewhat flattened. In their further growth 

 they either retain the primitive shape (fig. 

 622), or send out persistent lateral processes, 

 so forming nerve-cells or ganglion-globules; 

 or the processes of adjacent cells unite into 

 nucleated fibres, much resembling those of 

 the sympathetic system, in which the white 

 substance and axial fibre of the nerve-tubes 

 are formed as secondary deposits (fig. 523). 



In atrophy and degeneration of the ner- 

 vous elements, the nerve-cells become loaded 

 with fat and pigment, and the walls of the 

 nerve-tubes thinner, brittle, and the white 

 substance more or less replaced by granules 

 of fat. 



BiBL. Kolliker, Mik. Anat. 2; Todd, 

 Cycl. Anat. and Phys. iii.; Paget, Brit, and 

 For. Med. Rev, 1842. xiv. 



