I002 



HUMAN ANATOMY. 



844. 



Schmidt- 



Lantermann 



segment 



Axis-cylinder 



opposite each constriction (Ranvier). The neurilemma also suffers no break at 



the nodes, but is continuous from one segment to the other. 



In addition to the partial interruptions at the 

 nodes, the medullary sheath after treatment with 

 osmic acid frequently appears broken^ by clear 

 narrow clefts that extend obliquely from the neuri- 

 lemma to the axolemma and thus subdivide each 

 internodal segment into a number of smaller 

 tracts, known as the Schmidt- Lantermann segments 

 (Fig. 844). The oblique clefts do not all extend 

 in the same direction, even within the same inter- 

 nodal segment, since they are usually directed from 

 without inward and towards the nodal constrictions 

 and, therefore, have an opposed disposition at the 

 ends of the same as well as of the adjoining seg- 

 ments. The significance of this subdivision is un- 

 certain ; many regarding the details as artefacts. 

 According to Capparelli \ however, the apparent 

 clefts are in reality unstained membraneous septa 

 that pass obliquely from the axolemma to the inner 

 surface of the neurilemma and serve to hold the 

 axis-cylinder in place and to enclose the myelin. 

 The studies of Hatai ^ on the arrangement of the 

 neurokeratin seem to support these conclusions. 

 Within each internodal segment, beneath the sheath 

 of Schwann, lies a single (sometimes more than 

 one) small neurilemma-cell which consists of an 

 elongated oval nucleus surrounded by a meagre 

 amount of cytoplasm. These cells represent the 



remains of the formative elements {sheath-cells) that during the growth of the 



nerve-fibre were active in providing its envelope (page loii). 



Fig. S45. 



Cleft 



Node of Ranvier 



Medullated nerve-fibres after treatment 

 with osmic acid; A, fibre showing reticu- 

 lum within medullary coat ; B, one showing 

 same coat divided into segments. X 500. 



Medullated nerve-fibres becoming nonmedullated on approaching 

 their termination. ;■', 235. 



Depending upon the presence or absence of the medullary sheath throughout 

 the greater part of their course, nerve-fibres are distinguished as medullated or non- 



^Archivf. mikros. Anat. u. Entwick., Bd. 66, 1905. 

 * Journal of Comparative Neurology, vol. xiii., 1903. 



