NORMAL HISTOLOGY. 



FIG. 92. 



The neurilemma, sheath of Schwann, or primitive sheath, 

 the outer covering of the nerve-fibre, is a delicate, homogeneous, 

 elastic membrane, closely investing the medullary substance, and 

 resembling the sarcolemma. On its inner surface, placed at regular 

 intervals corresponding to the position of the nodes of Ranvier, are 

 the nerve-corpuscles, meagre accumulations of protoplasm sur- 

 rounding the oval nuclei. The medullated fibres of the white matter 

 of the brain and spinal cord, as well as those composing the optic 

 and acoustic nerves, are noteworthy as being without a neurilemma, 

 the surrounding neuroglia in these positions assuming the support 

 and covering of the fibres. 



The non-medullated, pale, or Remak's fibres, as indicated by 

 the first name, are devoid of medullary substance, consisting of the 

 axis-cylinder and the more or less modified neuri- 

 lemma ; such fibres, when aggregated, appear as 

 grayish, semi-transparent bands. While every 

 medullated nerve-fibre, before reaching its pe- 

 ripheral distribution, loses the white substance of 

 Schwann and becomes sooner or later a non- 

 medullated fibre, the nerves constituting the 

 sympathetic system especially represent this 

 group, and evince the distinctive tendency to 

 give off branches, which unite to form the char- 

 acteristic plexuses. The presence of both va- 

 rieties of fibres, however, in nerve-trunks is 

 quite usual ; a conspicuous example of this asso- 

 ciation is found in the vagus of the dog, where 

 large bundles of both kinds are included within 

 a common sheath. 



The fibrillse constituting the axis-cylinders of 

 non-medullated fibres are especially distinct, this 

 feature being probably due to the generous 

 amount of neuroplasm separating the fibrillae ; not infrequently local 

 accumulations of this interfibrillar substance occur, producing the 

 conspicuous varicosities seen along the course of the fibres. The 

 nerve-nuclei are far more numerous than in medullated fibres ; they 

 are, however, irregularly distributed, lying upon the surface of the 

 fibre and beneath the outer delicate sheath. This enveloping sheath 

 the attenuated representative of the neurilemma is often difficult 

 or impossible to distinguish, being very thin and closely adherent to 

 the fibre. The smallest nerve-fibrils are probably without this coat, 

 the fibrilke continuing as naked bundles, with the exception of the 

 imperfect covering afforded by the numerous overlying nerve-nuclei. 

 Non-medullated nerve-fibres are prone to form rich plexuses, the 



Non-medullated nerve- 

 fibres from the sympa- 

 thetic system : the nucle- 

 ated fibres join to form a 

 plexus. 



