624 STRUCTURE AND ARRANGEMENT OF THE NERVE ELEMENTS. 



means of transverse or oblique fibers that divide the myelin between two annular 

 constrictions into a number of successive segments. In this way are formed the 

 oblique clefts or indentations of Schmidt, Lantermann, and Kuhnt in the myelin, 

 as shown in Fig. 221. 



According to Leydig and Joseph the axis-cylinders contain a delicate reticular 

 framework, in the midst of which the fibrils of the axis-cylinder are embedded. 



The nerve-fibers are undivided in their course in the nerve-trunk. As they 

 approach their terminal distribution, they divide usually into two, less commonly 

 into several, fibers that undergo no further change. 



In animals the nerve-sheaths are sometimes still more complex. Thus, in the 

 electrical nerve of the electrical catfish the sheath of Schwann of the individual 

 nerve-fiber is constituted of such a large number of layers that the fiber may 

 attain the thickness of a knitting needle. In the invertebrates the nerve-fibers as 



FIG. 221. Medullated 

 Nerve-fiber Stained 

 Black by Osmic 

 Acid : fs, annular 

 constriction of Ran- 

 vier; sch, sheath of 

 Schwann (after Eich- 

 horst). 



FIG. 222. Transverse Section through a Portion of the Median Xcrvi 

 neurium; pe, perineurium ; ed, endoneurium (after Eichhorst). 



a rule have no medullary sheath. Retzius found them present, however, in the 

 shrimp. 



The connective tissue mixed with elastic fibers that surrounds a nerve-trunk 

 is designated epineurium (Fig. 222, <?/?) The individual nerve-fibers are united 

 in the nerve-trunk into bundles and each of the latter is surrounded by the peri- 

 neurium (pe) , from which the endoneurium (cd) passes inward between the nerve- 

 fibers. These sheaths are at the same time provided with concentric layers 

 constituted of smooth connective-tissue cells that can be stained with silver 

 nitrate. Even the individual nerve-fiber, which results from the breaking up of 

 a bundle of fibers, is further surrounded by a sheath composed of flat connective- 

 tissue cells in mutual contact (Henle's sheath). The longitudinal network of 

 blood-vessels of the nerve-fibers is supported by the connective tissue of the 

 latter. The lymphatics form spaces, which reach to the individual fibers. 



In their development the nerve-fibers first consist of fibrils that become sur- 

 rounded by connective-tissue and finally by medullary sheaths. At birth the 

 cerebral motor nerves and the auditory nerve alone are medullated. The longi- 



