. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. The Dominating Factor 669 Naked Axon Axon with Medullary Sheath Axon with Medullary Sheath and Neurolemma The neurolemma, composed of ectodermal cells derived from the original medullary tube, is a very thin, living sheath while the myelin sheath is a thicker, secreted layer which is white in color in fresh tissue. Probably the neurolemma secretes the myelin ma- terial, but only in the presence of the nerve fiber. Nervous tissue in which mye- linated fiber
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. The Dominating Factor 669 Naked Axon Axon with Medullary Sheath Axon with Medullary Sheath and Neurolemma The neurolemma, composed of ectodermal cells derived from the original medullary tube, is a very thin, living sheath while the myelin sheath is a thicker, secreted layer which is white in color in fresh tissue. Probably the neurolemma secretes the myelin ma- terial, but only in the presence of the nerve fiber. Nervous tissue in which mye- linated fibers predominate is white in color while cell-bodies and unmyelinated fibers, either naked or Remak's fibers, being composed of living protoplasm, form "gray ; In the case of fibers covered by both sheaths, the medullary sheath, but not the neurolemma, is interrupted at fre- quent intervals along the fiber, at the nodes of Ranvier (Fig. 595). Each in- ternodal ("between the nodes") region is associated with a single neurolemma cell containing a conspicuous nucleus. When the myelin sheath alone is pres- ent it is not segmented but forms instead a continuous tubular covering over the fiber. All these types of fibers and their sheaths are diagrammatically repre- sented in Figure 595. o â » *â¢. o Motor Plate 5. Microscopic Structure in Muscle Microscopic examination of a neuron after proper staining reveals structural details of great complexity. Neurofibrils, like myofibrils in a muscle cell, extend through the cell from end to end. They also surround the nucleus. In all probability they are highly elaborated parts of the cytoplasm directly concerned with the transmission of nerve impulses. Around the nucleus and in the dendrites, but not in the neurite, is found the so-called tigroid substance, or Nissl bodies, which apparently plays some part in the metabolism of the neuron, since it varies in amount in accordance with the amount of work done. Dend
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