108 



HISTOLOGY. 



of which has been modified to form the medullary sheath, and 

 the rest left unchanged as Schwann's corpuscles. Others 

 regard the medullary sheath as a product of the axis cylinder. 

 The function of the medullary sheath is mainly that of an 

 insulator. The irritability of the nerve increases during devel- 

 opment with the growth of the medullary sheath. 



FIG. 80. 



Mauthner'8 sheath 



Axis cylinder 



Neurofibrti* 



Ci'iiiciit xnhx1nrr 



'is cylinder 



Line of Schmidt- 

 Lantermann 



Rchwann's corpuscle 



Medullary sheath 



Schwann's sheath 



Diagram of the structure of a medullated nerve fibre, showing two different views 

 concerning the relations of the sheaths of Mauthner and of Schwann. Compare the right 

 and left sides. 



Medullated nerves usually have a layer outside Schwann's 

 sheath. This is of connective-tissue origin, and often shows a 

 fibrillary structure, but is in many cases homogeneous. It pos- 

 sesses always on its inner surface a number of flat epithelial 



