It has been shown that the niolecular architecture of the nerve axon sheath is qualitatively similar in all fiber types, whether vertebrate or invertebrate and whether highly myclinated or relatively unmyelinated. The essential feature of similarity is the presence in the sheath of lipide molecules oriented with long axes radial and of protein components oriented with long axes tangentlal, the two constituents forming a concentric layered structure (see Schmitt and Bear, '39). Polarization optical methods permit determination of the relative concentration of oriented lipide and protein constituents in the sheath of fresh fibers. Quantitative rneasureinents are available, however, only in the case of frog sciatic fibers (Schmitt and Bear, '37). I f the ultimate goal of such investigations, namely, the determination of the role of the sheath in nerve phenomena, is to be attained it is desirable that quantitative investigations be extended to include fiber types having widely different physiological as well as structural properties. With such data at hand it is to be hoped that correlations between optical and physical data will lead to suggestions as to the role of the sheath in conduction phenomena.
The birefringence of frog sciatic fibers has the value of 0.011 for the larger fibers. A t the other extreme of sheath organization, naniely that found in the so-called unmyelinated invertebrate fibers, quantitative measurements comparable t o those of the myelotropic vertebrate fibers are impossible because of the extreme thinness of the sheath. However, characteristics of the metatropic reaction suffice to indicate the order of magnitude of the birefringence for comparison with the more highly organized sheaths (Bear and Schmitt, '37 ; Bear, Schmitt and Young, '38). Between these two extremes of sheath types, there exists intermediate types called by Gothlin ('13) weakly myelotropic fibers. Examples of , 363