FEATURES OF THE NEURAL STRUCTURE 



57 



however, are commonly described, the medullated and the non-medul- 

 lated. In both of these the fiber consists of the essential fibrils embedded 

 probably in a clear substance called the neuroplasm, while according to 

 a few observers a delicate reticulum is also present in the fiber. 



In the medullated fibers the bundle of fibrils, often more or less flattened 

 and irregular, is surrounded by the axolemma and then by a relatively 

 thick covering of a highly refractive substance of a fatty nature called 

 myelin, this covering being the medullary sheath or myelin sheath. The 

 contained myelin is kept in place by a neurokeratin network. The 

 sheath is divided into segments by oblique fissures, the incisures of 

 Schmidt, the segments bearing the name of Lantermann. As usually 

 studied, the myelin sheath is colored black by osmic acid. In the periph- 

 eral nerves (mostly cerebrospinal, but also sympathetic in part) the 

 myelin sheath is in turn covered by a transparent structureless membrane 



FIG. 25 



Nerves of a mesenteric lymph-gland from a new-born dog. (Manoudlian.) 



called the neurilemma, or sheath of Schwann. At intervals varying with 

 the diameter of the fibril-bundle or fiber the myelin sheath is interrupted, 

 these interruptions being the nodes of Ranvier, and it is through these 

 nodes, apparently, that the fiber receives its nutriment from the lymph 

 and excretes its katabolic waste. They are from 80 to 900 mmm. apart, 

 these extremes corresponding to fiber-diameters of about 2 and 25 mmm. 

 respectively. Each of Lantermann's segments in man contains one 

 neurilemma-nucleus, surrounded by a small amount of protoplasm. 

 At the nodes the neurilemma or sheath of Schwann is thickened, being 

 thus continuous along the fiber, oftentimes from close to its "origin" in 

 the nerve-cell nearly to its termination. 



Non-medullated fibers (Remak's) lack the myelin sheath. Usually 

 they are surrounded by the neurilemma, and are then from about 4 to 



