Visual accommodation in vertebrates: mechanisms, physiological response and stimuli

The mechanism and stimulation of the accommodative reflex in vertebrate eyes are reviewed. Except for lampreys, accommodation is brought about by intraocular muscles that mediate either a displacement or deformation of the lens, a change of the corneal radius of curvature or a combination of these mechanisms. Elasmobranchs have little accommodation and are emmetropic in water rather than hyperopic as commonly stated. Accommodation in teleosts and amphibians is well understood and achieved by lens displacement. The accommodative mechanism of amniotes is of considerable diversity and reflects different lifestyles rather than phylogenetical relationships. In all amniotes, the ciliary muscle never has a direct impact on the lens. It relaxes the tension applied to the lens by zonular fibers and/or ligaments. In birds and reptiles the ciliary muscle is usually split into two parts, of which the anterior portion changes the corneal radius of curvature. The deformation of the lens is generally achieved either by its own elasticity (humans, probably other mammals and sauropsids) or by the force of circular muscle fibers in the iris (reptiles, birds, aquatic mammals). In the second part of the paper, some of the current hypotheses about the accommodative stimulus are reviewed together with physiological response characteristics.     

Corneal accommodation in chick and pigeon

Summary We have investigated the role of changes in corneal radius of curvature in effecting accommodation in the bird's eye. It was found that in natural accommodation (measured by IR photoretinoscopy) changes of corneal radius of curvature (measured by IR photokeratometry) play an important role in both the chick and the pigeon. In the adult pigeon the cornea is indeed responsible for the largest part of natural accommodation (up to approx. 9 D). In this animal the corneal diameter (as seen from the optical axis of the eye) decreases in accommodation which can be taken to explain the change of corneal radius of curvature. In the chicken, corneal accommodation is combined with other mechanisms (total accommodative range 15–17 D, corneal accommodation about 8 D). The chicken's cornea is aspherical within the pupil area leading to large measurement variation in photokeratometry if the Purkinje images are not symmetrical to the pupillary axis.Abbreviation IR infrared     

The action of ciliary muscle contraction on accommodation of the lens explored with a 3D model

Biomechanics and Modeling in Mechanobiology - The eye’s accommodative mechanism changes optical power for near vision. In accommodation, ciliary muscle excursion relieves lens tension,...     

Prey capture and accommodation in the sandlance, Limnichthyes fasciatus (Creediidae; Teleostei)

The eyes of the sandlance, Limnichthyes fasciatus (Creediidae, Teleostei) move independently and possess a refractive cornea, a convexiclivate fovea and a non-spherical lens giving rise to a wide separation of the nodal point from the axis of rotation of the eye much like that of a chameleon. To investigate this apparent convergence of the visual optics in these phylogenetically disparate species, we examine feeding behaviour and accommodation in the sandlance with special reference to the possibility that sandlances use accommodation as a depth cue to judge strike length. Frame-by-frame analysis of over 2000 strikes show a 100% success rate. Explosive strikes are completed in 50 ms over prey distances of four body lengths. Close-up video confirms that successful strikes can be initiated monocularly (both normally and after monocular occlusion) showing that binocular cues are not necessary to judge the length of a strike. Additional means of judging prey distance may also be derived from parallax information generated by rotation of the eye as suggested for chameleons. Using photorefraction on anaesthetised sandlances, accommodative changes were induced with acetylcholine and found to range between 120 D and 180 D at a speed of 600–720 D s⁻¹. The large range of accommodation (25% of the total power) is also thought to be mediated by corneal accommodation where the contraction of a unique cornealis muscle acts to change the corneal curvatures. Accepted: 8 December 1999     

Optical properties of a cephalopod eye (the short finned squid,Illex illecebrosus)

Summary The extent to which the cephalopod eye is optically similar to the teleost eye was determined by measuring refractive error, accommodative ability, spherical and chromatic aberrations, and refractive indices and radii of curvature of the ocular media. The squid eye is well corrected optically underwater although a tendency toward hyperopia exists. This may be due to the existence of chromatic aberration and the fact that an aquatic environment is somewhat limited to the blue end of the spectrum. Accommodation takes place by movement of the lens toward the retina in a manner similar to the teleost eye. However, the squid lens is not spherical but slightly flattened. The lens is overcorrected as far as spherical aberration is concerned. Thus peripheral light rays focus further from the lens than paraxial ones. The function of this unusual example of lens development is unknown.     

Sensitivity of Chaos Measures in Detecting Stress in the Focusing Control Mechanism of the Short-Sighted Eye

When fixating on a stationary object, the power of the eye’s lens fluctuates. Studies have suggested that changes in these so-called microfluctuations in accommodation may be a factor in the onset and progression of short-sightedness. Like many physiological signals, the fluctuations in the power of the lens exhibit chaotic behaviour. A breakdown or reduction in chaos in physiological systems indicates stress to the system or pathology. The purpose of this study was to determine whether the chaos in fluctuations of the power of the lens changes with refractive error, i.e. how short-sighted a subject is, and/or accommodative demand, i.e. the effective distance of the object that is being viewed. Six emmetropes (EMMs, non-short-sighted), six early-onset myopes (EOMs, onset of short-sightedness before the age of 15), and six late-onset myopes (LOMs, onset of short-sightedness after the age of 15) took part in the study. Accommodative microfluctuations were measured at 22 Hz using an SRW-5000 autorefractor at accommodative demands of 1 D (dioptres), 2 D, and 3 D. Chaos theory analysis was used to determine the embedding lag, embedding dimension, limit of predictability, and Lyapunov exponent. Topological transitivity was also tested for. For comparison, the power spectrum and standard deviation were calculated for each time record. The EMMs had a statistically significant higher Lyapunov exponent than the LOMs (\(0.64\pm 0.33\) vs. \(0.39\pm 0.20~\hbox {D}/\hbox {s}\)) and a lower embedding dimension than the LOMs (\(3.28\pm 0.46\) vs. \(3.67\pm 0.49\)). There was insufficient evidence (non-significant p value) of a difference between EOMs and EMMs or EOMs and LOMs. The majority of time records were topologically transitive. There was insufficient evidence of accommodative demand having an effect. Power spectrum analysis and assessment of the standard deviation of the fluctuations failed to discern differences based on refractive error. Chaos differences in accommodation microfluctuations indicate that the control system for LOMs is under stress in comparison to EMMs. Chaos theory analysis is a more sensitive marker of changes in accommodation microfluctuations than traditional analysis methods.     

A detailed paraxial schematic eye for the White Leghorn chick

We studied the normal ocular development of the chick (Gallus gallus domesticus, White Leghorn) up to 15 days of age using both longitudinal and cross-sectional methods. The change in refractive error, corneal curvature and axial ocular distances were used to construct schematic eyes. Equations are presented which allow prediction of refractive error changes associated with changes in vitreous chamber depth. The mean refractive error was +3.2 D at hatching, which reduced by 66% over the first 3 days and stabilized by 11 days of age. The lens thickened and the anterior chamber deepened from hatching, but vitreal elongation and corneal flattening were delayed until after the first 3 days, suggesting that normal eye growth may be initially inhibited or inactive during an initial emmetropization period, and subsequently activated in response to myopic defocus arising from the continually expanding lens. Finally, when compared with published data on other chick strains, we find differences in the degree of hyperopia at hatching due to differences in lens thickness. However, the rate of ocular and vitreal expansion and the developmental changes in corneal power are similar, making the schematic eyes presented here generally applicable to different strains of chickens.     

Volume change of the ocular lens during accommodation

During accommodation, mammalian lenses change shape from a rounder configuration (near focusing) to a flatter one (distance focusing). Thus the lens must have the capacity to change its volume, capsular surface area, or both. Because lens topology is similar to a torus, we developed an approach that allows volume determination from the lens cross-sectional area (CSA). The CSA was obtained from photographs taken perpendicularly to the lenticular anterior-posterior (A-P) axis and computed with software. We calculated the volume of isolated bovine lenses in conditions simulating accommodation by forcing shape changes with a custom-built stretching device in which the ciliary body-zonulae-lens complex (CB-Z-L) was placed. Two measurements were taken (CSA and center of mass) to calculate volume. Mechanically stretching the CB-Z-L increased the equatorial length and decreased the A-P length, CSA, and lens volume. The control parameters were restored when the lenses were stretched and relaxed in an aqueous physiological solution, but not when submerged in oil, a condition with which fluid leaves the lens and does not reenter. This suggests that changes in lens CSA previously observed in humans could have resulted from fluid movement out of the lens. Thus accommodation may involve changes not only in capsular surface but also in volume. Furthermore, we calculated theoretical volume changes during accommodation in models of human lenses using published structural parameters. In conclusion, we suggest that impediments to fluid flow between the aquaporin-rich lens fibers and the lens surface could contribute to the aging-related loss of accommodative power. lens volume calculation; intralenticular fluid movement; presbyopia; mammalian lens     

The eye of the hooded seal,Cystophora cristata,in air and water

Summary Multiple refractive state measurements were made on a male and female hooded seal (Cystophora cristata) when the eyes were exposed to air and to water. The measures, made by conventional retinoscopy and by photorefraction, show that the seals are moderately hyperopic (2–3 diopters) in water and moderately myopic (2–4 diopters) in air. No significant astigmatism was noted in either medium. The absence of refractive state variation over time suggests that an accommodative mechanism is insignificant or absent, although histological study indicates that the ciliary muscle is well developed.Photokeratoscopy, carried out on two animals with two keratoscopic instruments, show that the cornea is relatively flat (30 mm, or about one-half the diameter of the eye). Furthermore the cornea is only slightly astigmatic (less than 1 diopter). The refractive power of the external corneal surface (in air), calculated from a measurement of corneal refractive index of 1.378, amounts to only 10 or 11 diopters.As in the typical fish eye, hooded seal lenses are spherical or nearly spherical in shape (24–23 mm), and have short focal lengths (30–32 mm). Focal measures for rays at varying distances from the lens center indicate that spherical aberration is well corrected.There is no indication in this seal species, of a previously reported adaptation involving a highly astigmatic cornea which together with a slit pupil can minimize the optical effect of movement from water to air.     

Reversible Femtosecond Laser-Assisted Myopia Correction: A Non-Human Primate Study of Lenticule Re-Implantation after Refractive Lenticule Extraction

LASIK (laser-assisted in situ keratomileusis) is a common laser refractive procedure for myopia and astigmatism, involving permanent removal of anterior corneal stromal tissue by excimer ablation beneath a hinged flap. Correction of refractive error is achieved by the resulting change in the curvature of the cornea and is limited by central corneal thickness, as a thin residual stromal bed may result in biomechanical instability of the cornea. A recently developed alternative to LASIK called Refractive Lenticule Extraction (ReLEx) utilizes solely a femtosecond laser (FSL) to incise an intrastromal refractive lenticule (RL), which results in reshaping the corneal curvature and correcting the myopia and/or astigmatism. As the RL is extracted intact in the ReLEx, we hypothesized that it could be cryopreserved and re-implanted at a later date to restore corneal stromal volume, in the event of keratectasia, making ReLEx a potentially reversible procedure, unlike LASIK. In this study, we re-implanted cryopreserved RLs in a non-human primate model of ReLEx. Mild intrastromal haze, noted during the first 2 weeks after re-implantation, subsided after 8 weeks. Refractive parameters including corneal thickness, anterior curvature and refractive error indices were restored to near pre-operative values after the re-implantation. Immunohistochemistry revealed no myofibroblast formation or abnormal collagen type I expression after 8 weeks, and a significant attenuation of fibronectin and tenascin expression from week 8 to 16 after re-implantation. In addition, keratocyte re-population could be found along the implanted RL interfaces. Our findings suggest that RL cryopreservation and re-implantation after ReLEx appears feasible, suggesting the possibility of potential reversibility of the procedure, and possible future uses of RLs in treating other corneal disorders and refractive errors.     

Maintaining transparency: a review of the developmental physiology and pathophysiology of two avascular tissues

The lens and cornea are transparent and usually avascular. Controlling nutrient supply while maintaining transparency is a physiological challenge for both tissues. During sleep and with contact lens wear the endothelial layer of the cornea may become hypoxic, compromising its ability to maintain corneal transparency. The mechanism responsible for establishing the avascular nature of the corneal stroma is unknown. In several pathological conditions, the stroma can be invaded by abnormal, leaky vessels, leading to opacification. Several molecules that are likely to help maintain the avascular nature of the corneal stroma have been identified, although their relative contributions remain to be demonstrated. The mammalian lens is surrounded by capillaries early in life. After the fetal vasculature regresses, the lens resides in a hypoxic environment. Hypoxia is likely to be required to maintain lens transparency. The vitreous body may help to maintain the low oxygen level around the lens. The hypothesis is presented that many aspects of the aging of the lens, including increased hardening, loss of accommodation (presbyopia), and opacification of the lens nucleus, are caused by exposure to oxygen. Testing this hypothesis may lead to prevention for nuclear cataract and insight into the mechanisms of lens aging. Although they are both transparent, corneal pathology is associated with an insufficient supply of oxygen, while lens pathology may involve excessive exposure to oxygen.     

Visual optics in toads (Bufo americanus)

Aspects of visual optics were investigated in the American toad (Bufo americanus). The development of the refractive state of the eye during metamorphosis was followed with IR photoretinoscopy. Frozen sections documented the changes in optical parameters before and after metamorphosis. There is a difference in light sensitivity between juvenile and adult toads. Binocular accommodation in adult toads was observed. 1. IR photoretinoscopic measurements showed that the refractive state of the eye changed very rapidly during metamorphosis, about 10 D/h while the animal entered the terrestrial habitat. 2. Frozen sections showed that the almost spherical lens in a tadpole eye had flattened in a just metamorphosed toad's eye while at the same time the distance of the lens to the retina had decreased. However, the morphological measurements were not sufficiently sensitive to record the relatively small changes in ocular dimensions that were responsible for the rapid changes in refractive state during metamorphosis. 3. Schematic eyes, with homogeneous and non homogeneous lenses, were constructed for tadpoles, juvenile toads, and adult toads. 4. Nonparaxial raytracing studies in schematic eyes suggested that the lenses of animals of the three developmental stages tadpole, juvenile toad, and adult are not homogeneous but have a refractive index gradient. The raytracing studies indicated that the refractive index gradient is different for the different developmental stages, being highest in the tadpole lens. 5. The observations of toads during feeding behavior at different light levels showed an increased light sensitivity in the adult nocturnal toads in contrast to the juvenile animals, which are diurnal. The increased light sensitivity could partly be explained with an increase in aperture and an increase in red rod outer segments. To fully explain the higher light sensitivity in adult toads, changes in neuronal parameters had to be assumed. 6. Retinoscopic measurements of the resting refractive state in the adult toad showed a hyperopic defocus of about +8 D. By subtracting the measurement artefact for retinoscopy, the true resting focus was found to be nearly emmetropic. 7. The amount of natural accommodation in adult toads during normal feeding behavior was investigated with IR photoretinoscopy. Binocular accommodation of about 8 D was observed.     

The ageing lens and cataract: a model of normal and pathological ageing

Cataract is a visible opacity in the lens substance, which, when located on the visual axis, leads to visual loss. Age-related cataract is a cause of blindness on a global scale involving genetic and environmental influences. With ageing, lens proteins undergo non-enzymatic, post-translational modification and the accumulation of fluorescent chromophores, increasing susceptibility to oxidation and cross-linking and increased light-scatter. Because the human lens grows throughout life, the lens core is exposed for a longer period to such influences and the risk of oxidative damage increases in the fourth decade when a barrier to the transport of glutathione forms around the lens nucleus. Consequently, as the lens ages, its transparency falls and the nucleus becomes more rigid, resisting the change in shape necessary for accommodation. This is the basis of presbyopia. In some individuals, the steady accumulation of chromophores and complex, insoluble crystallin aggregates in the lens nucleus leads to the formation of a brown nuclear cataract. The process is homogeneous and the affected lens fibres retain their gross morphology. Cortical opacities are due to changes in membrane permeability and enzyme function and shear-stress damage to lens fibres with continued accommodative effort. Unlike nuclear cataract, progression is intermittent, stepwise and non-uniform.     

Optical characteristics of the eye of the flounder

Summary The winter flounder,Pseudopleuronectes americanus, is mildly hyperopic. However, chromatic aberration exists in significant amounts and therefore the eye may be emmetropic (zero refractive error) in natural conditions when light is restricted to shorter wavelengths. Large accommodative lens motion was observed along the direction of the pupil axis. This direction is rare among the teleosts and is the result of the unusual split origin of the retractor lentis muscle. While the lens is spherical, as in other teleosts, the retina is not uniformly distant from the lens. Rather, a vertical asymmetry exists such that dorsal and ventral portions of the retina are further from the lens than the central retina. In view of the existing large accommodative ability, this distortion of the globe is not likely to have an optical function but is probably due to the shape of the cartilagenous scleral cup supporting the eye in its extraorbital location. Further, the lens is overcorrected for spherical aberration so that rays passing through the periphery of the lens are focused further away. The value of a lens of this type is unclear.     

Focusing by shape change in the lens of the eye: a commentary on Young (1801) ‘On the mechanism of the eye’

In his Bakerian Lecture paper of 1801, Thomas Young provided the best account up to that time of the eye''s optical system, including refraction by the cornea and the surfaces of the lens. He built a device, an optometer, for determining the eye''s state of focus, making it possible to prescribe appropriate correction lenses. His main contribution, however, was to show that accommodation, the eye''s focusing mechanism, was not the result of changes to the curvature of the cornea, nor to the length of the eye, but was due entirely to changes in the shape of the lens, which he described with impressive accuracy. He was wrong, however, in believing that the reason the lens bulges when focusing on near objects was because it behaved as a contracting muscle. Half a century later, Helmholtz showed that the lens bulges not by its own contraction, but when it is relaxed as a result of contraction of newly discovered circular muscles in the ciliary body. This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society.     

Terrestrial optics in an aquatic eye: The sandlance,Limnichthytes fasciatus (Creediidae,Teleostei)

The sandlance, Limnichthyes fasciatus (Creediidae, Teleostei), behaves like a marine chameleon, with independent movements of its turret-like eyes, highly-effective camouflage and rapid strikes for isolated, mobile prey at close quarters. The optical system has a fixed circular pupil, a deep pit fovea and a flattened lens unlike any other teleost lens so far described. The convex, laminated structure of the cornea is also unparalleled in a teleost which suggests that the cornea may play a refractive role that might compensate for the reduced power of the flattened lens. This suggestion has been supported in the present investigation by four independent sets of observations:- i. Purkinje images formed underwater by the cornea; ii. Measurements of the magnification of intra-corneal iridophores viewed through the corneal lenticle; iii. Measurements of the magnification produced by the dissected corneal lenticle and lens when viewed over a grating; iv. Ray tracing experiments comparing the degree of refraction produced by the lens and by the corneal lenticle. All experimental observations confirm that the cornea of the sandlance has a significant refractive role, with a power of approximately 200 D compared with a power of 550 D for the lens. This is the first report of a significant refractive role played by the cornea in a teleost. The optical system of lens plus cornea, in combination with a deep pit fovea, may be more suitable for the detection and visual localisation of small, moving, underwater prey than the conventional wide-field spherical lens system of other teleosts. The evolutionary convergence of this marine optical system and lifestyle with those of the chameleon is remarkable, given the constraints imposed by underwater optics.     

Patient-specific modeling of corneal refractive surgery outcomes and inverse estimation of elastic property changes

The purpose of this study is to develop a 3D patient-specific finite element model (FEM) of the cornea and sclera to compare predicted and in vivo refractive outcomes and to estimate the corneal elastic property changes associated with each procedure. Both eyes of a patient who underwent laser-assisted in situ keratomileusis (LASIK) for myopic astigmatism were modeled. Pre- and postoperative Scheimpflug anterior and posterior corneal elevation maps were imported into a 3D corneo-scleral FEM with an unrestrained limbus. Preoperative corneal hyperelastic properties were chosen to account for meridional anisotropy. Inverse FEM was used to determine the undeformed corneal state that produced <0.1% error in anterior elevation between simulated and in vivo preoperative geometries. Case-specific 3D aspheric ablation profiles were simulated, and corneal topography and spherical aberration were compared at clinical intraocular pressure. The magnitude of elastic weakening of the residual corneal bed required to maximize the agreement with clinical axial power was calculated and compared with the changes in ocular response analyzer (ORA) measurements. The models produced curvature maps and spherical aberrations equivalent to in vivo measurements. For the preoperative property values used in this study, predicted elastic weakening with LASIK was as high as 55% for a radially uniform model of residual corneal weakening and 65% at the point of maximum ablation in a spatially varying model of weakening. Reductions in ORA variables were also observed. A patient-specific FEM of corneal refractive surgery is presented, which allows the estimation of surgically induced changes in corneal elastic properties. Significant elastic weakening after LASIK was required to replicate clinical topographic outcomes in this two-eye pilot study.     

NO EVIDENCE OF ACCOMMODATION IN THE EYES OF THE BOTTLENOSE DOLPHIN, TURSIOPS TRUNCATUS

Bottlenose dolphins ( Tursiops truncatus ) are aquatic mammals that must come to the surface to breathe. As a result, it might be expected that their eyes are adapted for both aerial and underwater vision. Earlier studies suggest that dolphins are emmetropic ( i. e. , focused at infinity) in water, and in some cases, emmetropic in air, although the mechanisms that permit these animals to see well in both media are not well understood. Nor is it known whether they can accommodate to focus sharply on objects at different distances. We employed video photoretinoscopy to investigate the possibility of an active accommodative mechanism in the eyes of the bottlenose dolphin in water. Measurements of the refractive state in water indicated near emmetropia for two individuals and slight myopia (nearsightedness) for the third individual. No clear cases of accommodation were observed underwater in any of the subjects examined. Vision underwater may be used to supplement echolocation. If so, such a role might not require an accommodative mechanism.     

Biometry of Anterior Segment of Human Eye on Both Horizontal and Vertical Meridians during Accommodation Imaged with Extended Scan Depth Optical Coherence Tomography

Purpose

To determine the biometry of anterior segment dimensions of the human eye on both horizontal and vertical meridians with extended scan depth optical coherence tomography (OCT) during accommodation.

Methods

Twenty pre-presbyopic volunteers, aged between 24 and 30, were recruited. The ocular anterior segment of each subject was imaged using an extended scan depth OCT under non- and 3.0 diopters (D) of accommodative demands on both horizontal and vertical meridians. All the images were analyzed to yield the following parameters: pupil diameter (PD), anterior chamber depth (ACD), anterior and posterior surface curvatures of the crystalline lens (ASC and PSC) and the lens thickness (LT). Two consecutive measurements were performed to assess the repeatability and reproducibility of this OCT. They were evaluated by calculating the within-subject standard deviation (SD), a paired t-test, intra-class correlation coefficients (ICC) and the coefficient of repeatability/reproducibility (CoR).

Results

There were no significant differences between two consecutive measurements on either horizontal or vertical meridians under both two different accommodative statuses (P>0.05). The ICC for all parameters ranged from 0.775 to 0.998, except for the PSC (0.550) on the horizontal meridian under the non-accommodative status. In addition, the CoR for most of the parameters were excellent (0.004% to 4.89%). In all the parameters, only PD and PSC were found different between the horizontal and vertical meridians under both accommodative statuses (P<0.05). PD, ACD, ASC and PSC under accommodative status were significantly smaller than those under the non-accommodative status, except that the PSC at the vertical meridian did not change. In addition, LT was significantly increased when accommodation.

Conclusion

The extended scan depth OCT successfully measured the dimensions of the anterior eye during accommodation with good repeatability and reproducibility on both horizontal and vertical meridians. The asymmetry of lens posterior surface and oval-shaped pupil were found during accommodation.     

Biofeedback of accommodation to reduce functional myopia

Functional myopia may be defined as the refractive condition of the eye due to spasm of the ciliary muscle. As a result of the ciliary muscle spasm, the crystalline lens becomes more convex, creating a myopic refractive condition. The normal increase and decrease in the refractive power of the crystalline lens is known as accommodation and is controlled by the autonomic nervous system innervation to the ciliary muscle. Previous studies have reported that voluntary control of accommodation is possible by biofeedback training (Cornsweet & Crane, 1973; Randle, 1970). The present research investigated the application of biofeedback control of accommodation to reduce functional myopia. A double-reversal, multiple-baseline design was used to conduct the experiment. The results revealed that the three adult male subjects achieved the preset criterion, a 1/2-diopter reduction from initial baseline to a subsequent baseline. Further analysis of the data revealed even greater changes between initial baseline and feedback periods. Although generalization to a nonexperimental environment was not trained, each subject showed a reduction in myopia and an increase in visual acuity. The results of the experiment clearly demonstrated that functional myopia is subject to voluntary control.The research presented in the text was submitted in partial fulfillment of the requirements for the degree of doctor of philosophy in the Ferkauf Graduate School, Yeshiva University. The author wishes to express appreciation to his dissertation committee, professors Carl Auerbach, Abraham Givner, and Allan C. Goldstein. Also, appreciation is given to Ralph Dippner and John Orzuchowski for their assistance.