An American text-book of physiology . logically controlled by a nucleus which becomes situated aboutmidwav between its extremities. The cell-substance is largelv transformed intomyelin, and the line of junction between two of these sheathing cells forms anode of the nerve-fibre. In the sheath of a growing nerve-cell at least twochanges are clearly marked: As the axis increases in diameter the medul-lary sheath becomes thicker. The change is such that in the peripheralsystem the areas of the axis-cylinder and of the medullary sheath as shownin cross sections remain nearly equal (Fig. 149). On t
An American text-book of physiology . logically controlled by a nucleus which becomes situated aboutmidwav between its extremities. The cell-substance is largelv transformed intomyelin, and the line of junction between two of these sheathing cells forms anode of the nerve-fibre. In the sheath of a growing nerve-cell at least twochanges are clearly marked: As the axis increases in diameter the medul-lary sheath becomes thicker. The change is such that in the peripheralsystem the areas of the axis-cylinder and of the medullary sheath as shownin cross sections remain nearly equal (Fig. 149). On the other hand the lengthof the internodal segments tends to increase with an increase in the diameterof the nerve-fibre, and for nerves of the same diameter it is less in man thanin the lower mammals. In a given fibre the segments are. shorter at the extremeperipheral end (Key and ). In the young fibres, also, they are shorterand increase in length with age. Howell, Budgett, and Leonard: Journal of Physiology, 1894, toI. 616 AN A mi: RICA N TEXT-BOOK OF PHYSIOLOOY. A jiliysiological significance attaches to these scgmeuts, because, as Ranvierlong since pointed out, it is at the nodes that various staining reagents most Iasily reach the axis-cylinder. This sug-gests that normal nutritive exchanges mayfollow the same path and thus short inter-nodal segments giving rise to many nodeswould represent tlie condition most favor-able to exchange between the axis and the 1 ;i;.—l,..u,L;ii,,u ^y;. ami tr!(,-l) p i- . i • sections of nerve-fibrcs. The lu-avy border SUrrOUndmg plasma. illUS lar, histologl- represents the medullary sheath, which be- ^^^ observation showS the more numcrOUScomes thicker in the larger fibres. Human sciatic nerve. X 200 diameters (modified from nodcS where the pliysiological proCCSSeS van Gehuchtcn). ^^.^ presumptivelv most active, and hence supports the hypothesis suggested. Cases of the interjiolation of new sheath-ing cel
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