THE NERVE FIBER 



189 



covering is found is one in which a single layer of these sheath cells 

 have united by their edges to form an unbroken tubular sheath some- 

 times called the neurolemma. The nuclei of these cells appear as if they 

 lie on the outside of the sheath. They do not do this, however, as the 

 cell-substance can be seen, with higher powers, covering the nucleus 

 as is done in all cells. 



In the higher vertebrate animals there is found on some nerve fibers, 

 in addition to this connective-tissue sheath, another and inner covering 

 of an entirely different nature. This is a layer of a thick oily or fatty 



FIG. 166. A, two non-medullated nerve fibers from a large connective in the Octopus. The 

 left-hand fiber is a section. That on the right is a surface view showing the delicate, branch- 

 ing connective cell which covers it. Fibrillation not shown. B, a group of nerve fibers and 

 parts of two other groups in the developing optic nerve of a three-months-old human embryo. 



substance called myelin. It forms a complete layer around parts of the 

 fiber only (Fig. 167). As to its origin, its position between the nerve 

 process and the surrounding sheath leaves it to be decided as to which 

 of these two have produced it. As the neurite is a highly specialized struc- 

 ture which is probably expending all of its energies in the work of conduc- 

 tion, it is probable that the other, the sheath, is the producer of the mye- 

 lin, and that it secretes it much as some other connective-tissue cells, the 

 fat-cells, secrete the fat, by the metabolic activity of their cytoplasm. 

 Whether this secretion is a superficial one, leaving the myelin between 

 the inner surface of the sheath and the outer surface of the neurite, or 

 whether it is an intra-cellular secretion of the sheath cell in which each 



