ANATOMY AND PHYSIOLOGY OF NERVOUS SYSTEM. (j():} 



known as the incisurcs of Lantermann, and the segments as those of 

 Schmidt. 



Neurilemma. The neurilemma or sheath of Schwann, sur- 

 rounds the medullary sheath to form the outer boundary of the nerve- 

 fiber. It is a thin, elastic, very delicate, hyaline, and transparent mem- 

 brane. It is comparable to the cell-wall of a cell. Between the neu- 

 rilemma and medullary sheath there are irregularly scattered ovoid 

 nuclei. They are the nerve-corpuscles, and are analogous to the 

 muscle-corpuscles previously mentioned. Each nerve-corpuscle is sur- 

 rounded by a thin zone of protoplasm. 



Between the myelin layer and the neurilemma is a thin zone of 

 protoplasm. AY hen this arrives at the level of the annular constric- 

 tions it is reflected upon itself to line the internal surface of the 

 myelin layer (Man timer's membrane). The protop asm is also insinu- 

 ated into the incisures of Lantermann and decomposes the layer of 

 myelin into the superposed segments of Schmidt. 



Nodes of Ranvier. At intervals of about one micromillimeter 

 along the course of the nerve there appear constrictions: the nodes 

 of Ranvier. At these points the myelin sheath is interrupted so that 

 the neurilemma appears to do the constricting. That portion of. the 

 nerve-fiber between any two constrictions is termed an internodal seg- 

 ment. At about the center of each internodal segment is located one, 

 sometimes more, nerve-corpuscles. 



Such is the composition of a medullated nerve-fiber. This type 

 of nerve is found chiefly in the white matter of the nerve-centers and 

 in the cerebro-spinal nerves, with the exception of the olfactory nerve. 



NONMEDULLATED NERVE-FIBERS. They occur especially in the 

 sympathetic system, but are also present to a slight extent in the 

 cerebro-spinal nerves. 



Each fiber consists of a bundle of fibrils primitive fibrils 

 which are inclosed in a delicate, transparent, and elastic sheath. The 

 fibrils are very delicate and somewhat flattened. Here and there along 

 the course of the fibrils will be found oval nuclei. These latter lie 

 between the axis-cylinders and their enveloping neurilemma. As 

 these fibrils contain no myelin, they are not blackened by osmic acid. 

 This allows of a differentiation between medullated and nonmedul- 

 lated nerves when examining the nerve-supply of a tissue. 



NERVE-TRUNKS consist of bundles of nerve-fibers. Each bundle, 

 of course, contains a greater or less number of fibrils. Several bun- 

 dles are held together by a common connective-tissue sheath: the 

 epineurium. Delicate fibrils lie between the nerve-fibers, to constitute 



