or, 



GENERAL PRINCIPLES OF ZOOLOGY. 



fcij 

 B 



fibrillse to tlio ganglion cell, in which they spread out and pass 

 over into other processes. The branching of neiirites and 

 dendrites is a separation of the contained fibrillee; the ganglion 

 cell, the place of exchange of fibrillar between the various jirocesses. 

 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 fibrillar, distinguished from muscle 



fibrilte by the absence of cross- 

 striation; from connective -tissue 

 fiibrillai by the ease with which they 

 are injured; in preserved material 

 they frequently swell and show vari- 

 cosities (fig. 5-3). M'any fibrillse 

 united in a bundle form a nerve- 

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

 gray nerve-fihre in distinction from 

 the ivliite or meduJlated fibres. In 

 the latter the fibre or axis-cylinder 

 is surrounded by a medullary slieath 

 (fig. 54, Z/) composed of myelin, a fat- 

 like substance, blackened by osmic 

 acid and separated into variously 

 shaped ' myelin drops. ' The medul- 

 lary sheath ajij^ears to act as an 

 insulator. 



Both niedullated and non-med- 

 ullated fibres can be enclosed in a 

 A, without, B, -with sheath of ' sheath of Schwann. " This is a 

 Schwann. (From Hatschek., 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 (ficj. 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., Liiinbricus), arthropods, and molluscs, and then 

 chiefly ii^ the peripheral nervous system. In the ganglia (the 

 nervous centres of the last three grou])s) the ganglion cell usually 

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

 vided with lateral brandies or dendrites (fig. 74). The medullary 



B 



Fig. 54. 



Fig. 55. 



Fio. .53. — Nerve flbriUfi? with varicosi 



tie.s. (From Hatschek.) 

 Fig. .54. — .'Non-mediillated 



Fig. 55.— Medullatecl 



nerve-fibres, 

 of ' 



