. Chordate anatomy. Chordata; Anatomy, Comparative. HISTOLOGY 99 the fiber, it seems to be nearly or quite interrupted so that the neurilemma there comes into close relation with the nerve fiber (Fig. giB). The neuraxon therefore presents a segmented appearance due to these nodes of Ranvier. Nerves whose individual fibers possess the myelin sheath appear more nearly white than do non-medullated nerves. The so-called " white " parts of the brain and spinal cord consist mainly of medullated nerves. Non-medullated fibers and the cell-bodies of neurons are the chief con- stituents of &qu
. Chordate anatomy. Chordata; Anatomy, Comparative. HISTOLOGY 99 the fiber, it seems to be nearly or quite interrupted so that the neurilemma there comes into close relation with the nerve fiber (Fig. giB). The neuraxon therefore presents a segmented appearance due to these nodes of Ranvier. Nerves whose individual fibers possess the myelin sheath appear more nearly white than do non-medullated nerves. The so-called " white " parts of the brain and spinal cord consist mainly of medullated nerves. Non-medullated fibers and the cell-bodies of neurons are the chief con- stituents of "gray ; The sheaths doubtless serve for the protection, insulation and nutrition of the nerve fiber. The source of the myelin is not definitely known. Bundles of nerve fibers. Epineurium. Perineurium. Endoneurium. Fig. 95.—Structure of a nerve. The figure represents a small part of a transverse section of a large nerve constituted of many bundles of medullated fibers. X20. (From Bremer, "Text-book of ;) A nervous organ is constituted of neurons supported by connective tissues accompanied by vascular tissues. In the brain and spinal cord of vertebrates occurs not only the usual mesenchymal connective tissue but another which is unique in that its cells have ectodermal origin in common with the nerve cells. Some of the cells of this neuroglia possess branched processes which make them confusingly similar in appearance to nerve cells. The neuroglia cells form, by means of their processes, a supporting network for the nerve cells. A nerve is a bundle of neuraxons, each of which may be ensheathed as described above, and all wrapped together within a sheet of connective tissue, the perineurium (Fig. 95) extensions of which (endoneurium) may penetrate into the bundle. Larger nerves consist of several or many bundles all tied together by connective tissue and enwrapped by a rela- tively thick epineurium. Small blood-vessels traverse the connective- t
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