GENERAL HISTOLOGY 



85 



B 



pulse from the cell; the dendrite is the afferent tract. The unipolar 

 nerve cell is also to be regarded as bipolar, its processes leaving the 

 cell from a common point, running together a short distance and then 

 branching like the letter T- This conception is intelligible if the 

 fibrillae be regarded as the carriers of stimuli, each process con- 

 sisting of a varying numljer of fibrilla;. These enter the cell, where 

 they cross similar fibril. ;e coming from 

 other processes, and then are dis- 

 tributed to the various neurites and 

 dendrites. The branching of a fibre 

 is thus a gradual distribution , of the 

 constituent fibrilla, and the cell is the 

 place where the e.xchange of fibrilla; 

 between the different processes occurs. 

 Thus it is immaterial whether two 

 bundles of fibrillar leave the cell at 

 different points or whether they are 

 united for a distance like a cable. 



In the central nervous system of 

 vertebrates the most minute elements 

 are the nerve fibril'a;, distinguished 

 from muscle fibrillre by the absence of 

 cross-striation; from connective-tissue 

 fibrillte by the ease with which they 

 are injured; in preserved material they 

 frequently swell and show varicosities 

 (fig. 54). Many fibrillfe united in a 

 bundle form a nerve-fibre (fig. 55, A) 

 which is called a gray nerve-fibre in 

 distinction from the white or niedullated fibres. In the latter the fibre or 

 axis-cylinder is surrounded by a medullary sheath (fig. 55, B) composed 

 of myelin, a strongly refractive fat-like substance, which appears to act 

 as an insulator. 



Both medull-ated and non-medullated fibres can be enclosed in a sheath 

 of Schwann. This is a feature of the peripheral nervous system and is 

 lacking in brain and spinal cord. It is a delicate envelope with nuclei 

 here and there (fig. 56). At times it has constrictions which cut through 

 the medullary sheath to the axis-cylinder (constrictions of Ranvier) . 



Multipolar and bipolar ganglion cells also occur in the invertebrates, 

 most commonly in the ccelenterates (fig. 57), more rarely in worms, 

 arthropods, and molluscs, and then chiefly in the peripheral nervous 



iillil 

 A 



B 



Fig. 54 Fig. 55 



Fig. 56. 



Fig. 54. — Xerve fibrilla; with vari- 

 cosities (from Hatschek). 



Fig. 55. — ,4, Non-medullated; B, 

 nerve-fibres. 



Fig. 56. — /I, Non-medullated; B, 

 meduUated nerve-fibres, with sheath 

 of Schwann (from Hatschek). 



