GENERAL PRINCIPLES OF ZOOLOGY. 



fibrillae to the ganglion cell, in which they spread out and pass 

 over into other processes. The branching of neurites and 

 dendrites is a separation of the contained fibrillse; the ganglion 

 cell, the place of exchange of fibrillae between the various processes. 

 Hence the ganglion cell is not a simple cell, but a cell plus plasma 

 products. 



The similar fibrillar structure of nerve-fibres has long been 

 known. In the central nervous system of vertebrates the most 

 minute elements are the nerve fibrillae, distinguished from muscle 



fibrillae. by the absence of cross- 

 striation ; from connective - tissue 

 fibrillae by the ease with which they 

 are injured; in preserved material 

 they frequently swell and show vari- 

 cosities (fig. 53). Many fibrillae 

 united in a bundle form a nerve- 

 fibre (fig. 54, A) which is called a 

 gray nerve-fibre in distinction from 

 the white or medullated fibres. In 

 the latter the fibre or axis-cylinder 

 is surrounded by a medullary sheath 

 (fig. 54, B) composed of my elm, a fat- 

 like substance, blackened by osmic 

 acid and separated into variously 

 shaped ( myelin drops. ' The medul- 

 lary sheath appears to act as an 



FIG. 53. FIG. 64. F.G. 55. insulator. 



FIG. 53. Nerve flbriiise with varicosi- Both medullated and non-med- 



ullated fibres can be enclosed in a 

 'sheath of Schwann.' This is a 

 feature of the fibres composing the 

 peripheral nervous system and is lacking in brain and spinal cord. 

 It is a delicate envelope with nuclei here and there (fig. 55). At 

 times it forms constructions which cut through the medullary 

 sheath to the axis-cylinder (nodes of Ranvier). 



Multipolar and bipolar ganglion cells also occur in the inverte- 

 brates, most commonly in the coelenterates (fig. 56), more rarely in 

 worms (e.g., Lumbricus), arthropods, and molluscs, and then 

 chiefly in the peripheral nervous system. In the ganglia (the 

 nervous centres of the last three groups) the ganglion cell usually 

 gives rise to a single strong process, which, however, is richly pro- 

 Tided with lateral branches or dendrites (fig. 74). The medullary 



B 



ties. (From Hatsohek.) 

 FiG.54. Non-medullated (_ Q _.. Q fiv ,^ 

 FIG. 55.-Medullated j- nerve-fibres, 

 A, without, B, with sheath of 

 Schwann. (From Hatschek.) 



