Nervous System 



367 



of the spinal cord, when regenerating through 

 a connective tissue gap, even over great dis- 

 tances, tend to retain their fascicular iden- 

 tity (Hooker, '30; Holtzer, '52b). Evidently, 

 such selectivity of association implies two 

 things: first, that nerve fibers of different 

 functional designations are constitutionally 

 (i.e., substantially) different, and second, 

 that they can recognize and distinguish one 

 another according to kind. 



Fasciculation occvirs by non-detachment 

 among fibers that have made contact, rather 

 than by active association. It therefore is 

 promoted by conditions furthering the 

 chances of contact. These chances are low 

 in compact tissues containing innumerable 

 separate fibrous guide lines, but high in 

 tissues with large liquid spaces, where the 

 nerve pathways are crowded into the rela- 

 tively few land bridges. Judging from tis- 

 sue cultures, established nerve fibers may 

 themselves bring about the latter condition 

 by liquefying surrounding colloids (Weiss, 

 '34a) and thus facilitate bundling. Moreover, 

 liquefaction around attached nerve fibers 

 will cause their floating stems to cling to- 

 gether, according to the same principle that 

 makes wet threads stick together in air, thus 

 assembling them secondarily into trunks. 

 These considerations furnish a ready expla- 

 nation of the fact that fibers tend to remain 

 separate in the gray matter, in the periph- 

 eral tissues and in nerve scars, whereas they 

 are aggregated into bundles in the more 

 liquid-filled interstices, particularly along 

 the blood vessels. However, much remains 

 still to be found out about the mechanisms 

 of fasciculation; particularly the factors ef- 

 fecting the sorting and collecting of fascicles 

 into still larger assemblies by wrappings of 

 connective tissue are still wholly unexplored. 

 Association with Sheath Cells; Myeliniza- 

 tion. The specific affinity between Schwann 

 cells and axons appears to be mutual. Sheath 

 cells attach themselves to nerve sprouts 

 (Speidel '32). The tips of nerve fibers, con- 

 versely, show a definite predilection for 

 strands of Schwann cells (Nageotte, '22; 

 Dustin, '17), and it is doubtful whether 

 naked sprouts, not enveloped by sheath cells, 

 can persist over appreciable distances. The 

 two cell types, when in contact, thus seem 

 to form firm unions, which actively resist 

 separation (Abercrombie, Johnson and 

 Thomas, '49). Sheath cells have been seen 

 to shuttle freely between nerve fibers of 

 different kinds (Speidel, '33), with a certain 

 preference for transfer from unmyelinated to 

 myelinated ones (Speidel, '50). Sheath cells, 



however, do not share the specificity of the 

 nerve fibers which they coat; this can be 

 inferred from the fact that regenerating 

 sensory or motor nerve sprouts penetrate 

 Schwann cords of either nerve type with 

 equal ease (see p. 362). 



Myelin is presvimably formed in the sur- 

 face of the axon, with the sheath cells (or, 

 in the central nervous system, the glia cells) 

 furnishing some essential stimulus or com- 

 plement (Speidel, '33, '35), but the details 

 of the process are not known. Since the same 

 sheath cell has been observed either to induce, 

 or fail to induce, myelin formation, depend- 

 ing on whether it joined the branch of a 

 potentially myelinated or a potentially un- 

 medullated fiber (Speidel, '33), the differ- 

 ential faculty for myelinization must be a 

 property of the nerve fiber itself. The lami- 

 nated structure of the myelin sheath (see 

 p. 347) and the proportionality between its 

 thickness and the caliber of the axon suggest 

 that sviccessive layers are shed by the sur- 

 face as the axon grows in width (see Geren 

 and Raskind, '53). 



As Schwann cells line up in tandem along 

 the axons, myelin formation progresses in 

 a general proximodistal sequence, starting 

 in each cell from the region of the nucleus. 

 The length of the cell defines a myelin 

 segment, and the junction of two cells, the 

 node of Ranvier. The standard length of 

 individual segments (internodes) amounts 

 to about 300 micra, which agrees with the 

 average length of an extended Schwann 

 cell in tissue cvxlture. The same average length 

 is found in regenerated nerve fibers regard- 

 less of diameter (Hiscoe, '47; Vizoso and 

 Young, '48). The greater internodal length 

 in primary (non-regenerated) fibers, which 

 varies directly with fiber diameter attain- 

 ing up to about 1500 micra, is presumably 

 to be ascribed to passive elongation by the 

 stretching of the nerves in tow of growing 

 organs (Hiscoe, '47; Young, '50). The longest 

 fibers having undergone the greatest ex- 

 tension would also end up with the longest 

 segments, and since the longest fibers 

 (within the same class) have the largest 

 caliber (Kolliker, 1896), a general propor- 

 tionality between fiber diameter and inter- 

 nodal length would result (Thomas and 

 Young, '49), which fact has recently assumed 

 added significance in connection with the 

 saltatory theory of nerve conduction. 



Saturation Factors. Volume and density of 

 peripheral innervation vary relevantly from 

 organ to organ and from region to region 

 within the same systems (skin, intestine. 



