. Analysis of development. Embryology; Embryology. Nervous System NERVE REGENERATION Since our insight into nerve development and growth has been greatly aided by studies on nerve regeneration, the essentials of this phenomenon may be briefly recapitulated here for later reference; for fuller reviews, see Cajal ('28), Nageotte ('22), and Boeke ('35). 349 freezing, chemical damage, etc.), the seg- ment lying distally to the lesion (the "distal" or "peripheral" stump), within a few days loses conductivity, and the individual nerve fibers in it become converted into non-con- d
. Analysis of development. Embryology; Embryology. Nervous System NERVE REGENERATION Since our insight into nerve development and growth has been greatly aided by studies on nerve regeneration, the essentials of this phenomenon may be briefly recapitulated here for later reference; for fuller reviews, see Cajal ('28), Nageotte ('22), and Boeke ('35). 349 freezing, chemical damage, etc.), the seg- ment lying distally to the lesion (the "distal" or "peripheral" stump), within a few days loses conductivity, and the individual nerve fibers in it become converted into non-con- ducting plasmatic strands ("Schwann cords" or "Buengner's cords"); myelin and axis cylinder remnants break down (o, Fig. 126B). and as they are being resorbed, their place. PROXIMAL DISTA L Fig. 126. Diagram of regeneration of single nerve fiber after transection (explanation in text). Nerve regeneration is the restoration of morphological and physiological continuity in a transected nerve. The older supposition that this may take place per primam fu- sionem of the severed ends has proved un- tenable; the observation that cut ends of axons in tissue culture that lie within a dis- tance of a few micra may merge during the first hours after severance (Levi, '34), does not apply to nerves in the body, where this condition is practically never realized. In the body, the nerve is restituted by renewed outgrowth of fibers from the proximal stump, repeating with some modifications the process of embryonic outgrowth. Briefly, the events are as follows (Fig. 126). Aft(!r a nerve has been severed or other- wise locally disrupted (, by pressure, is taken by the hypertrophying and multiply- ing sheath cells of Schwann {s, Fig. 126C). This combination of regressive and prolifera- tive processes is generally referred to as "Wallerian ; In the "proximal" or "central" stump, it remains confined to the immediate vicinity of the l
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