| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Structural Biophysics Group, Department of Optometry and Vision Sciences, Cardiff University, Cardiff, United Kingdom
Correspondence: Address reprint requests to Keith M. Meek, Redwood Bldg., King Edward VII Ave., Cathays Park, Cardiff University, Cardiff CF10 3NB, UK. Tel.: 44-292-087-6317; E-mail: meekkm{at}cf.ac.uk.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
|---|
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
|---|
; Farrell and McCally, 2000; Sivak, 1988).
The cornea not only refracts most of the incident light, but it also transmits >95% of this light. Corneal transparency has been the subject of much study over the years (Maurice, 1957; Hart and Farrell, 1969; Smith, 1969; Feuk, 1970; Benedek, 1971; Twersky, 1975; Worthington, 1984; Freund et al., 1986, 1995). It is now generally accepted that transparency depends on the destructive interference of light scattered away from the forward direction and that this, in turn, requires a certain amount of short-range ordering of collagen fibril positions (Hart and Farrell, 1969; Farrell and McCally, 2000). In Farrell's model, the scattering cross section per unit length for an isolated fibril, 
, may be expressed as (Farrell and McCally, 2000):
 | (1) |
s is the mass density of the stroma, is the number of fibril axes per unit area in a cross-section cut, Ro is the refractive increment (the change in refractive index with solute concentration), ffs is the volume fraction occupied by the hydrated fibrils in the stroma, and Mc and Mg are the mass fractions of dry collagen in the fibrils and biomolecules in the extrafibrillar matrix, respectively. From this equation it is clear that the scattering cross section depends on nf, ne, and hence m, i.e., the refractive indices of the hydrated collagen fibrils, of the extrafibrillar matrix, and their ratio.
Apart from the uniform refractive index model (Smith, 1969), all other explanations of corneal transparency assume that there is a significant difference in the values of nf and ne. Unfortunately, the two components (collagen fibrils and extrafibrillar matrix) cannot easily be isolated and examined in their physiological state, so it is not possible to obtain direct measurements of their refractive indices accurately (Maurice, 1957). Instead, their values must be estimated from known physical and chemical properties of the stroma and its constituents. Maurice (1957) found the refractive index of dry collagen, nc, to be 1.55, and went on to calculate nf as 1.47 and ne as 1.345. In a later article, these values were refined to nf = 1.51 and ne = 1.345 (Maurice, 1969).
Worthington (1984) used Gladstone and Dale's law of mixtures together with known values of the relative weights and densities of the corneal components to calculate the refractive indices. This was later refined by Leonard and Meek (1997) to give values of nf = 1.416 and ne = 1.356 for bovine corneal stroma. These were close to the values for human corneas reported by Freund and co-workers (Freund et al., 1995; nf = 1.407 and ne = 1.352). However, all the methods mentioned above rely on assumptions, many of which are now known to be incorrect. With measurements of volume fractions computed directly from x-ray diffraction data, Leonard and Meek (1997) used Gladstone and Dale's law to calculate the refractive indices for four species. They also reported average values from 40 species of nf = 1.416 and ne = 1.359. Very little difference in refractive indices was found between the species studied.
There is much uncertainty as to the mechanism by which light scattering increases as the cornea swells. Several changes occur in the tissue, some of which probably affect scattering more than others. For example, the hydrations of the various components are altered to differing degrees and their refractive indices change accordingly. The fibrils move further apart so that the phase difference between the various scattered waves is altered, changing scattering cross sections (Farrell and McCally, 2000). The number density and volume fraction of the fibrils both decrease. Freund et al. (1986) have produced a robust method for calculating corneal transmission from normal and swollen corneas, and have tested the method on slightly swollen corneas (up to 25%; Freund et al., 1991). However, to precisely relate changes in structure to changes in transparency as the cornea swells, it is necessary to know how the extra water is distributed (inside/outside fibrils, within "lakes"; Meek et al., 1991; Huang and Meek, 1999), how the collagen fibril arrangement changes (Freund et al., 1991; Meek and Quantock, 2001), and how the refractive indices change. The purpose of the present article is to address this last issue. Only when we know how the refractive indices change as fluid enters the stroma can we model how this affects light scattering (Benedek, 1989).
In this article we develop a simplified theoretical model of the corneal stroma consisting of hydrated, pseudo-hexagonally packed collagen fibrils embedded in a homogeneous, hydrated matrix. First, we apply Gladstone and Dale's law of refractive indices to calculate the volume fraction occupied by solvent in the physiological stroma. Along the way, we calculate values for a number of important structural parameters in the cornea. In the second part of the article, we apply the same law to determine a relationship between the refractive index of the whole stroma, and later the refractive index of the extrafibrillar matrix, as a function of tissue hydration. In the final part of the article, we compare the results of the theoretical variation of stromal refractive index versus hydration, with experimentally measured values.
Gladstone and Dale's law of mixtures applied to stroma at physiological hydration
According to Gladstone and Dale's law, the refractive index of a composite may be expressed as the partial sum of the refractive indices of its components n1, n2, ... nN, each weighted by the volume fraction occupied by that component, f1, f2, ... fN (Maurice, 1957; Worthington, 1984):
 | (2) |
, their Eqs. 9 and 10; however, the equations have been slightly modified from those in Leonard and Meek, 1997, by replacing the refractive index of water, 1.333, by the refractive index of salt solution, 1.335):
 | (3) |
 | (4) |
 | (5) |
|
|
),
 | (6) |
is the partial specific volume of collagen, Mc is the molecular weight of collagen, D is the collagen axial periodicity, pm is the center-to-center lateral spacing of collagen molecules within a fibril, 
is the packing angle of the collagen molecules, and NA is Avogadro's number. Substituting known values of these parameters and the value of pm obtained from wide-angle x-ray diffraction, Leonard and Meek (1997) obtained the value fcf = 0.37.
Leonard and Meek (1997) considered the stroma to consist of "unit cells" representing the average volume occupied by each fibril (thus neglecting the contribution of keratocytes to the stromal volume). With this model, they estimated the volume fraction per unit length of fibrils in the stroma, ffs, from the interfibrillar center-to-center Bragg spacing, pi, and the fibril diameter, a, both of which can be measured from low-angle x-ray diffraction patterns from the cornea (Gyi et al., 1988; Meek and Leonard, 1993), using
 | (7) |
The factor 1.12 relates the Bragg spacing from a liquidlike arrangement of fibrils (Worthington and Inouye, 1985) to the equivalent mean center-to-center spacing of the fibrils in a pseudo-hexagonal lattice. For bovine cornea at physiological hydration they calculated a value of ffs = 0.32 ± 0.08. Although the uncertainty was rather large, the mean value was close to the average value of the fibril volume fraction from 40 species (ffs = 0.28 ± 0.03). Using the calculated values of fcf, fcs, and ffs specific for cow (Table 1), we can substitute into Eqs. 3 and 4 to obtain nf = 1.413 and ne = 1.359.
By compartmentalizing the hydrated stroma into hydrated collagen fibrils, dry extrafibrillar matrix, and extrafibrillar solvent, we can, for completeness, calculate the volume fractions of fibrillar and nonfibrillar fluid, although for the purposes of later arguments, separation into fibrillar and nonfibrillar compartments is not really necessary. Gladstone and Dale's law may be written as
 | (8) |
 | (9) |
We therefore know values of all the terms in Eq. 8 except np, which can thus be determined, giving the value np = 1.485. The uncertainties in the values of ffs and fews imply that this figure has a precision of better than 4%.
The next step is to consider the compartmentalization of the solvent and the collagen within each fibril. Using the fact that fcf + fwf = 1, we see that fwf = 0.63. This value is written in Table 1.
Finally, we can calculate the volume fraction of intrafibrillar solvent in the stroma (fiws) by using the fact that fiws = fwf x ffs. This value, fiws = 0.20, is also presented in Table 1.
Gladstone and Dale's law applied to swollen corneas
Dependence of stromal refractive index on tissue hydration
Since the volume fractions of intrafibrillar and extrafibrillar solvent in the stroma are 0.20 and 0.58 respectively (Table 1), the total volume fraction of solvent in the stroma, fws, is 0.78. If the cornea swells such that the volume of solvent increases by a factor P to (1 + P) x its initial value, then to a good approximation its hydration (weight of water/dry weight) also increases by a factor P (since the mass of the additional ions introduced is negligible compared with the mass of the stroma).
In this section, we will use the convention that primed notation refers to the values of parameters in the swollen cornea. Thus using V to represent the initial volume of the stroma, Vw for the initial volume of solvent in the stroma and V' for the new (swollen) volume of the stroma, we can write
 | (10) |
 | (11) |
With the value for fws given above, we get
 | (12) |
). If the additional solvent therefore does not go into the fibrils, the refractive index of the fibrils remains unchanged but the extra solvent in the extrafibrillar space will cause a reduction in the value of ne. Furthermore, because the volume of the stroma is now greater, the various stromal volume fractions will change. The purpose of this section is to determine what changes occur in the volume fractions and the refractive indices, and hence to derive an expression for the expected change in the refractive index of the stroma as the tissue swells.
Equation 12 states that when the stroma swells, its volume increases by the factor 0.78 P. The volume fractions can be defined in terms of the new volume of the fibrils, V'f, and the new volume of the swollen stroma as follows:
 | (13) |
Since we assume the fibrils do not swell (Meek et al., 1991), the new volume of the fibrils, V'f, is the same as the original volume of the fibrils, Vf, and substituting from Eq. 12,
 | (14) |
 | (15) |
 | (16) |
With these new volume fractions, we can apply Gladstone and Dale's law to the swollen stroma:
 | (17) |
 | (18) |
 | (19) |
 | (20) |
 |
Using the known value Hphys = 3.2 and substituting into Eq. 19 gives:
 | (21) |
, where no uncertainty was quoted. However, other measurements, including those from the present work, suggest this figure is accurate to >±0.2%. Our value for the volume fraction of solvent in the stroma was derived using some values for which the uncertainty was also not quoted. The only other determination we have seen for this quantity comes from Worthington (1984) who quotes 0.819 (again without a precision estimate), which differs from our value by 5%. Using these uncertainties, we have estimated the precision with which our theoretical relationship (Eq. 21) is known, and these confidence limits are included in Fig. 1.
|
), nf will remain unchanged (i.e., n'f = nf). To calculate the dependence of n'e on P, we start by applying Gladstone and Dale's law to the extrafibrillar matrix,
 | (22) |
But f'pe = f'ps / f'es where volume fractions are defined in Table 1, and from Eq. 15 and the known value of fps,
 | (23) |
 | (24) |
We can thus use Eqs. 23 and 24 to determine f'pe:
 | (25) |
 | (26) |
 | (27) |
|
 | (28) |
|
Samples
Fresh bovine eyeballs were obtained from the abattoir and the corneal discs were excised from the eyes within 3 h of death. The endothelium and epithelium were removed by scraping with a scalpel and the epithelium side tagged using cotton. The corneas were wrapped in clingfilm and left at 4°C until needed.
Tissue equilibrium
The individual corneas were placed in 14-kDa cutoff dialysis tubing, which was carefully smoothed to ensure no air bubbles were trapped inside. Each piece of dialysis tubing was clamped at both ends and then placed into an equilibration solution containing a fixed concentration of polyethylene glycol. Buffer equilibrium solutions of Na2HPO4/NaH2PO4 at pH 7.1 were used and NaCl was added as required to reach a final ionic strength (µm) of 0.03 as previously described by Huang and Meek (1999). Concentrations of 0, 0.5, 0.75, 1.0, 1.5, 2.0, 2.5, 2.7, 3.0, and 3.5% polyethylene glycol (20 kDa, BDH Ltd., Warwickshire, England) were used to adjust the hydration of the tissues (Meek et al., 1991). The refractive index of the swollen tissue was then measured as described below. The corneas were reweighed to allow an average hydration during the course of the experiment to be calculated. They were then placed in an oven at 60°C until a constant dry weight was obtained.
Tissue hydration (H) was calculated using the following equation:
 |
Refractometry
A bench-top Abbe 60 Series Refractometer (Bellingham and Stanley Ltd., Tunbridge Wells, England) was used for the experiment. This was calibrated using a silica test plate of known refractive index, supplied with the instrument, and the calibration was checked using a series of sugar solutions of known refractive index. The instrument was standardized before each experiment by adjusting the illumination to give a clear black/white boundary from distilled water. Transmitted illumination was from a bench lamp and reflected illumination from an in-built LED light source to observe the critical angle. This resulted in one side of the field of view from each cornea appearing black and the other white. All measurements were made at room temperature.
The cornea was placed on the refractometer stage tag-side-(anterior stroma)-up and the refractive index measured by adjusting the LED light source until a good borderline quality was observed. The cornea was then placed posterior-side-up and the refractive index was measured in the same way.
The average refractive index measurements from anterior and posterior stroma as a function of tissue hydration are shown in Fig. 4 a, and the predicted relationship (Eq. 21) is superimposed for comparison. A Pearson linear correlation analysis (Fig. 4 b) yielded a significant positive correlation of 0.78 (p < 0.01) between the experimental points and their corresponding theoretical values.
|
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
|---|
used their versions of Eqs. 3 and 4 to calculate the values nf = 1.413 and ne = 1.357. Corneal keratocytes are thought to occupy 
10% of the stromal volume (Kaye, 1969) and this was neglected in Leonard and Meek's calculations based on their x-ray diffraction data. However, if we assume the refractive index of the cells matches that of the extrafibrillar space, we can reapply Gladstone and Dale's law, taking into account the volume occupied by the keratocytes. If 10% of the stromal volume is not available to the fibrils, the fibrillar volume fraction is reduced from 0.32 to 0.29 and the volume fraction of the extrafibrillar material is increased from 0.68 to 0.71. From Eqs. 3 and 4, the effect of these changes is to leave the value of nf unaltered, but to reduce the calculated value of ne to 1.358. This represents <0.1% change in the value of the refractive index when the presence of cells is taken into account. For this reason it was decided to neglect the effects of keratocytes in the present work.
The refractive index of a polymer solution (such as the extrafibrillar matrix of the corneal stroma), ne, can be expressed in terms of the specific refractive increment of the constituent proteins (Ro) and on their concentration (c):
 | (29) |
), so its concentration is 1.06 x 0.147, which equals 0.156 gm/ml. The value of Ro for most proteins is 0.18 ml/gm (Farrell and McCally, 2000). Substituting for these values, Eq. 29 gives ne = 1.362 (taking the values of all parameters to three decimal places in the calculations). Within the precision of the volume fractions quoted in Table 1, we regard this to be in reasonable agreement with the value of ne = 1.359 calculated in the present work.
We have shown both from a theoretical standpoint and experimentally how the average refractive index of the corneal stroma is reduced as the tissue swells. The agreement between experiment and theory, though showing a significant correlation, is clearly not exact (Fig. 4). Inspection of the distribution of data points in Fig. 4 a suggests that there is a shallower slope in the experimental data compared to the experimental curve. It is well-known that the anterior stroma swells very little in vitro (Müller et al., 2001), so its refractive index will not change much as the cornea as a whole swells. Conversely, most of the swelling takes place below these anterior layers, so changes in tissue hydration should primarily be reflected in changes in the refractive index of the posterior lamellae (Patel et al., 2000). This being the case, it is interesting to plot the swelling data for the anterior and the posterior stroma separately (Fig. 5). Despite the scatter in the experimental data points, it is evident that the theoretical expression fits the posterior swelling data (Fig. 5 a) better than the anterior data (Fig. 5 b), as expected.
|
) which, in turn, may be expected to alter the refractive index in a way not predicted by the current analysis. However, the contribution of the nonaqueous fraction of the extrafibrillar matrix to the refractive index is relatively small (the matrix is very hydrated and becomes more so as the tissue swells) and it will thus make only a small contribution to the refractive index.
In this article we have derived a simple expression that relates the refractive index of the corneal stroma to the increase in the volume fraction of solvent as the tissue swells (Eq. 19). From this it can be seen that only two parameters are required to specify the new refractive index, the refractive index of the stroma at physiological hydration and the volume fraction of solvent in the physiological stroma. We have calculated our value for the latter (78%) from the volume fractions of a number of other constituents, and it is in good agreement with the value of 77.2% estimated from the chemical composition of the stroma (Leonard and Meek, 1997). The change in refractive index with corneal swelling was previously studied by Fatt and Harris (1973), who produced a formula relating corneal refractive index to corneal thickness. However, their equation is asymptotic to ns = 1.342, whereas, at very large hydrations, the refractive index of the stroma must approach that of the solvent, as does our Eq. 19.
In the bovine cornea, the refractive index is different on the epithelial and endothelial sides, as previously reported for human and porcine corneas (Patel et al., 1995; Watanabe and Uozato, 2001). This refractive gradient was shown by Patel and co-workers to have no practical importance in terms of the power of the normal cornea and may simply arise from the differential hydration between the anterior and posterior cornea (Castoro et al., 1988).
Changes in the refractive index of the extrafibrillar matrix as the cornea swells would be expected to affect light scattering. According to Eq. 1, an increase in m, the ratio of the refractive indices of the fibrils and the matrix, would cause scattering to increase, and it is possible that this contributes to the increased light scattering observed in swollen corneas. The actual effect of such a change is difficult to assess because the swelling will be accompanied by changes in other parameters on which transmission depends, such as order in the packing of the fibrils and their number density. However, if we consider the effects of changes in refractive index alone, assuming these other parameters remain constant, the transparency model of Freund et al. (1986) predicts that the change in refractive indices between H = 3.2 and H = 8 reported here would cause an 5% increase in light scattering (Leonard, 1996; Meek et al., 2003), considerably less than the increase actually observed. It appears, therefore, that changes in refractive index of the extrafibrillar material make only a small contribution to the observed increase in light scattering when the cornea swells.
|
ACKNOWLEDGEMENTS |
|---|
|
TOPABSTRACTINTRODUCTIONDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
|---|
This work was funded by a program grant from the Medical Research Council (G0001033) and a refurbishment grant from the Royal Society/Wolfson Foundation.
Submitted on January 20, 2003; accepted for publication July 7, 2003.
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
|---|
Benedek, G. B. 1989. The molecular basis for corneal transparency. In Proceedings of the Corneal Biophysics Workshop. Corneal Biomechanics and Wound Healing. L. A. McNicol, and E. Strahlman, editors. National Eye Institute, Bethesda, MD. 1–15.
Castoro, J. A., A. A. Bettelheim, and F. A. Bettelheim. 1988. Water gradients across bovine cornea. Invest. Ophthalmol. Vis. Sci. 29:963–968.
Farrell, R. A., and R. L. McCally. 2000. Corneal transparency. In Principles and Practice of Ophthalmology. D. M. Albert and F. A. Jakobiec, editors. W. B. Saunders: Philadelphia, PA. 629–643.
Fatt, I., and M. G. Harris. 1973. Refractive index of the cornea as a function of its thickness. Am. J. Optom. Physiol. Opt. 50:383–386.
Feuk, T. 1970. On the transparency of the stroma in the mammalian cornea. IEEE Trans. Biomed. Eng. BME. 17:1866–1890.
Freund, D. E., R. L. McCally, and R. A. Farrell. 1986. Direct summation of fields for light scattering by fibrils with applications to normal corneas. Appl. Optics. 25:2739–2746.
Freund, D. E., R. L. McCally, R. A. Farrell, S. M. Cristol, N. L. L'Hernault, and H. F. Edelhauser. 1995. Ultrastructure in anterior and posterior stroma of perfused human and rabbit corneas: relation to transparency. Invest. Ophthalmol. Vis. Sci. 36:1508–1523.
Freund, D. E., R. L. McCally, and R. A. Farrell. 1991. Light scattering tests of structure in normal and swollen rabbit corneas. Johns Hopkins APL Tech. Digest. 12:137–143.
Gyi, T. J., K. M. Meek, and G. F. Elliott. 1988. The interfibrillar spacings of collagen fibrils in the corneal stroma: a species study. Int. J. Biol. Macromol. 10:265–269.
Hart, R. W., and R. A. Farrell. 1969. Light scattering in the cornea. J. Opt. Soc. Am. 59:766–774.[Medline]
Huang, Y., and K. M. Meek. 1999. Swelling studies on the cornea and sclera: the effects of pH and ionic strength. Biophys. J. 77:1655–1665.
Katz, E. P., and S. T. Li. 1973. The intermolecular space of reconstituted collagen fibrils. J. Mol. Biol. 73:351–369.[Medline]
Kaye, G. I. 1969. Stereologic measurements of cell volume fraction of rabbit corneal stroma. Arch. Ophthalmol. 82:792–794.[Medline]
Leonard, D. W. 1996. The ultrastructure of the corneal stroma and its implications for transparency. Ph.D. Thesis. The Open University, Milton Keynes, UK.
Leonard, D. W., and K. M. Meek. 1997. Refractive indices of the collagen fibrils and extrafibrillar material of the corneal stroma. Biophys. J. 72:1382–1387.
Maurice, D. M. 1969. The cornea and sclera. In The Eye. H. Davson, editor. Academic Press, New York. 489–599.
Maurice, D. M. 1957. The structure and transparency of the corneal stroma. J. Physiol. 136:263–286.
Meek, K. M., and A. J. Quantock. 2001. The use of x-ray scattering techniques to determine corneal ultrastructure. Prog. Ret. Eye Res. 20:95–137.[Medline]
Meek, K. M., and D. W. Leonard. 1993. Ultrastructure of the corneal stroma: a comparative study. Biophys. J. 64:273–280.
Meek, K. M., N. J. Fullwood, P. H. Cooke, G. F. Elliott, D. M. Maurice, A. J. Quantock, and R. S. Wall. 1991. Synchrotron x-ray diffraction studies of the cornea with implications for stromal hydration. Biophys. J. 60:467–474.
Meek, K. M., D. W. Leonard, C. J. Connon, S. Dennis, and S. Khan. 2003. Transparency, swelling and scarring in the corneal stroma. Eye. In press.
Müller, L. J., E. Pels, and G. F. J. M. Vrensen. 2001. The specific architecture of the anterior stroma accounts for maintenance of corneal curvature. Br. J. Ophthalmol. 85:437–443.
Patel, S., J. L. Alió, and J. J. Pérez-Santonja. 2000. A model to explain the difference between changes in refraction and central ocular surface power after laser in situ keratomileusis. J. Refract. Surg. 16:330–335.[Medline]
Patel, S., J. Marshall, and F. W. Fitzke, III. 1995. Refractive index of the human corneal epithelium and stroma. J. Refract. Surg. 11:100–105.[Medline]
Quantock, A. J., K. M. Meek, P. Brittain, A. E. A. Ridgway, and E. J.-M. A. Thonar. 1991. Alteration of the stromal architecture and depletion of keratin sulphate proteoglycans in oedematous human corneas: histological, immunochemical and x-ray diffraction evidence. Tissue Cell. 23:593–606.[Medline]
Sivak, J. G. 1988. Corneal optics in aquatic animals: how they see above and below. In The Cornea: Transactions of the World Congress on the Cornea. H. D. Cavanagh, III, editor. Raven Press Ltd., New York. 181–186.
Smith, J. W. 1969. The transparency of the corneal stroma. Vision Res. 9:393–396.
Twersky, V. 1975. Transparency of pair-correlated, random distributions of small scatterers, with applications to the cornea. J. Opt. Soc. Am. 65:524–530.[Medline]
Watanabe, C., and H. Uozato. 2001. Distribution of refractive index of the cornea. Invest. Ophthalmol. Vis. Sci. 42:S899.
Worthington, C. R. 1984. The structure of cornea. Quart. Rev. Biophys. 17:423–451.[Medline]
Worthington, C. R., and H. Inouye. 1985. X-ray diffraction study of the cornea. Int. J. Biol. Macromol. 7:2–8.
This article has been cited by other articles:
 |
|
 |
|
  D. G. Dawson, T. R. Kramer, H. E. Grossniklaus, G. O. Waring III, and H. F. Edelhauser Histologic, Ultrastructural, and Immunofluorescent Evaluation of Human Laser-Assisted In Situ Keratomileusis Corneal Wounds Arch Ophthalmol, June 1, 2005; 123(6): 741 - 756. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
