GENERAL HISTOLOGY. 115 



the mode of action of a ganglion-cell is understood only 

 when, on the one hand, one or numerous paths of stimu- 

 lation converge to it, while, on the other, a process 

 serves for further transmission of the stimuli onward. 

 Probably in the case of the " apolar ganglion-cells" 

 all, in case of the unipolar most, of the processes 

 were destroyed by unsuitable methods of preparation. 

 Recent investigations have justified this suspicion, since 

 multipolar and bipolar ganglion-cells have been demon- 

 strated by isolation in case of the caieuteratcs (Fig. 50) and 

 by methods of staining in crustaceans and worms. 



The Nerve-fibres likewise are best known in the ver- 

 tebrates (Figs. 51-53). The fundamental elements of these 

 are very fine fibres, the nerve-fibrils, distinguished from 

 muscle-fibrils by an absence of cross-striations, from con- 

 nective-tissue fibrils by their great liability to injury. 

 Even in good preparations they show a tendency to form 

 minute swellings, the varicosities. Many parallel-running 

 nerve-fibrils constitute a nerve-fibre, which are called gray 

 nerve-fibres, in distinction from a second sort, the white or 

 mednllatcd. In the medullated nerve-fibre, the fibre itself, 

 the axis-cylinder, is surrounded by a layer of nerve-marrow 

 or myelin, a fatlike substance which blackens strongly in 

 osmic acid, is very refractive, and easily decomposed into 

 variously shaped drops, the myelin drops. The medullary 

 sheath seems to act like an insulator. 



Finally, unmedullated and medullated nerve-fibres may 

 be surrounded by Schwann's sheath. This is found in 

 all the nerve-fibres which run outside of the brain and spinal 

 cord, and is just as constantly absent in the nerve-fibres 

 within the central organs. Schwann's sheath is a delicate, 

 structureless covering in which here and there nuclei are 

 embedded. At greater intervals are found indentations, 

 which extend through the medulla, quite to the axial fibres 

 (the nodes of Ranvier). 



In the invertebrates we meet simpler conditions. Here 

 are commonly seen only nerve-fibrils, which in greater or 

 less number combine directly for the formation of nerves. 



