ONE FIGUREI n a previous paper (Schmitt, Bear and Clark, '35, hereinafter referred to as I) the diffraction patterns of a variety of nerves were described, and it was shown that the patterns of all the fresh nerves studied were due to the myelin sheath. Characteristic long-spacing equatorial reflections which are orders of a fundamental spacing of 171 A were shown to be due to lipide molecules whose long chains are directed radially in the sheath. The ineridionally accentuated shortspacing ring at 4.7 A was attributed to the interchain distance of these lipides in the tangential direction.Since publication of this paper considerable additional diffraction data have been obtained and further information has come from polarization optical studies. The newer information has necessitated revision of earlier views of myelin sheath structure in some details, and progress reports have been incorporated in recent reviews (Schmitt, '36 ; Schinitt and Bear, '39; Schmitt, '39; Schmitt and Palmer, '40). The conclusioit was reached that the myelin sheath, like tubular myelin forms, is composed of cylindrical smectic fluid-crystalline lipide layers wrapped concentrically about the axon. The sheath is more cornplex than such myelin forms in having thin sheets of neurokeratinogenic protein intercalated between lipide layers. For a more precise description detailed information was required concerning the configurations and dimensions of the lipide molecules as pure components and as mixtures both in the dry and wet condition. This information has now been at least partially supplied (Bear, Palmer and Schmitt, '41; Palmer and Schmitt, '41) and in the present paper the structure of the sheath is discussed in the light of these new facts. E X P E R I M E N T A L METHOD

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Since the early work of Valentin (1861) much information about the ultrastructure of nerve has been obtained by means of polarization optics (for reviews, see Schmidt, '24, '34).Most of this work has concerned itself with the structure of the myelin sheath. The optical properties of the myelin sheath lipoids were interpreted by Klebs (1865) as indicating the presence of positive uniaxial crystallites oriented with optic axes extending radially. Viewed laterally medullated fibers appear, therefore, negatively birefringent with respect to their long axes. This negativity is a fairly certain criterion of the presence of myelin in any types of nerve fibers which show it.Ambronn and Held (1895) compared the osmic acid and Weigert techniques with that of the polarization optics and found the latter far superior for the detection of myelin and for revealing the finer nuances of organization of the myelin. Recent work from Diamare's laboratory (see particularly Cristini, '28, and Mezzino, '31) and by Schmidt ( '35, '36) has directed attention to the role of the protein elements in determining the structure of the myelin sheath. The high degree of organization of this sheath is shown perhaps most strikingly by the x-ray diffraction analysis (Schmitt, Bear and Clark, '35). Almost without exception the past work on nerve birefringence has been of a purely qualitative nature.It is the purpose of the present papers to provide a quantitative basis for this evaluation of the physiological role of myelin and sheath structures.

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Summary 1. In avoiding certain inherent indeterminacies in classical morphological methods and in obtaining further details regarding the microscopic and ultra‐microscopic structure of nerve axon sheaths, the methods of polarization optics and X‐ray diffraction are of great value. In the case of the myelin sheaths of vertebrate nerve fibres, for example, the optical and diffraction studies indicate the structure of the living fibre's sheath to be of smectic mixed fluid‐crystalline nature. The structure is, therefore, readily altered by chemical treatment to form the artifacts commonly observed in histological preparations. 2. A number of considerations suggest that the specific configuration of the lipoid and protein components of the myelin sheath is as follows. The proteins occur as thin sheets wrapped concentrically about the axon, with two bimolecular layers of lipoids interspersed between adjacent protein layers. While this means that in a radial direction within the cylindrical sheath there are alternate predominantly aqueous and predominantly hyirocarbon phases, the latter cannot be described as being entirely “non‐aqueous” 3. Polarization optical studies show that, contrary to the general view, invertebrate nerve fibres quite widely possess, aside from connective tissue investments, thin sheaths which are essentially similar in ultrastructure to the well‐defined myelin sheaths of vertebrate fibres. The demonstration of this fact involved a reinterpretation of the meaning of Gothlin's metatropic reaction, in which immersion of the fibre in media of high refractive index permits the (intrinsic) birefringence of lipoids present in the normal sheath in an oriented condition to become apparent by the reduction of the masking (form) double refraction of protein. Associated with the invertebrate metatropic axon sheaths are cells similar to the Schwann cells of vertebrate fibres. 4. Quantitative birefringence studies have disclosed that the axon sheaths of a wide variety of fibre types differ chiefly with respect to the relative amounts of oriented protein and lipoid present. This difference is observed not only between typical invertebrate and vertebrate fibres, but also when the fibres of a single vertebrate nerve are compared. For example, the curve obtained when sheath birefringence of frog sciatic fibres is plotted against fibre diameter shows wide variations in the magnitude of double refraction, changing continuously from birefringence due preponderantly to lipoids, in the case of the larger fibres, to that which, in the smallest fibres, results primarily from proteins. The transition from lipoid to protein predominance occurs at a fibre diameter of about 2μ., agreeing well with the division between “medullated” and “non‐medullated” fibres arrived at by histologists. It has been suggested that the low concentration of lipoid in the sheaths of small fibres is related to physical factors opposing the introduction of the lipoids into cylindrical structures of high curvature. 5. Examination of available inform...