Mechanisms of accommodation in the bird eye

Summary Keratoscopic study of corneal curvature before and after accommodation in two common bird species failed to provide evidence of a corneal accommodative mechanism. Accommodative changes in refractive state measured retinoscopically are presumably brought about by the effect of ciliary muscle contraction on lens curvature. However, retinoscopic and freeze-sectioning study of accommodation in diving ducks supports the long suspected existence of an iris accommodative mechanism capable of producing dramatic changes in lens curvature. This mechanism is believed to be a means of compensating for the refractive loss of the cornea in water.This research was supported in part by a grant from the Canadian National Sportsmen's Fund. The assistance of Dr. W.F. Long and Mr. R. Ortleib (Niska Wildlife Foundation) is gratefully acknowledged.     

Determination of 19 elements in human eye lenses

Neutron activation analysis was used to determine the concentrations of 19 elements in normal and senile human cataractous lenses. It was found that the concentrations of Ca, Na, Cl, Eu, Sb, and Fe were significantly higher, and those of K, Rb, Cs, Cr, Mn, Co, Sc, and Ce were significantly lower in senile mature cataractous lenses than those in normal human eye lenses. No changes were found for the concentrations of Se, Zn, Mg, S, and Th in the two groups. Positive correlations between Na, Cl, and Ca and K, Rb, and Cs were found, whereas a significantly negative correlation between na, Ca, Cl and K, Rb, Cs were found. The roles of these elements in the evolution of cataract are discussed.     

Steady-state analysis of self-heated thermistors using finite elements

The purpose of this work is to validate, using numerical, finite element methods, the thermal assumptions made in the analytical analysis of a coupled thermistor probe-tissue model upon which a thermal conductivity measurement scheme has been based. Analytic, closed form temperature profiles generated by the self-heated thermistors can be found if three simplifying assumptions are made: the thermistor is spherical; heat is generated in all regions of the bead; and heat is generated uniformly in the bead. This analytic solution is used to derive a linear relationship between tissue thermal conductivity and the ratio of thermistor temperature rise over electrical power required to maintain that temperature rise. This derived, linear relationship is used to determine thermal conductivity from the observed experimental data. However, in reality, the thermistor bead is a prolate spheroid surrounded by a passive shell, and the heating pattern in the bead is highly nonuniform. In the physical system, the exact relationship between the tissue thermal conductivity and parameters measured by the thermistor is not known. The finite element method was used to calculate the steady-state temperature profiles generated by thermistor beads with realistic geometry and heating patterns. The results of the finite element analysis show that the empirical, linear relationship remains valid when all three simplified assumptions are significantly relaxed.     

Validation of composite finite elements efficiently simulating elasticity of trabecular bone

Patient-specific analyses of the mechanical properties of bones become increasingly important for the management of patients with osteoporosis. The potential of composite finite elements (CFEs), a novel FE technique, to assess the apparent stiffness of vertebral trabecular bone is investigated in this study. Segmented volumes of cylindrical specimens of trabecular bone are compared to measured volumes. Elasticity under uniaxial loading conditions is simulated; apparent stiffnesses are compared to experimentally determined values. Computational efficiency is assessed and recommendations for simulation parameters are given. Validating apparent uniaxial stiffnesses results in concordance correlation coefficients 0.69 ≤ r_𝒸 ≤ 0.92 for resolutions finer than 168 μm, and an average error of 5.8% between experimental and numerical results at 24 μm resolution. As an application, the code was used to compute local, macroscopic stiffness tensors for the trabecular structure of a lumbar vertebra. The presented technique allows for computing stiffness using smooth FE meshes at resolutions that are well achievable in peripheral high resolution quantitative CT. Therefore, CFEs could be a valuable tool for the patient-specific assessment of bone stiffness.     

A fluid-structure interaction problem in biomechanics: prestressed vibrations of the eye by the finite element method.

The object of this work has been to develop a mechanical and numerical model of the eye submitted to vibrations, and in particular, to calculate the influence of intraocular pressure (IOP) on the eye resonance frequencies. Our mechanical model of the eye relies upon the theory of the mechanics of continuous media. The numerical model results from a model analysis of the vibrations of the eye using a finite element method (FEM) for discretization. The eye can be schematically represented as a prestressed shell, filled by an inviscid barotropic compressible fluid, which leads us to formulate and solve a problem of vibrations of a coupled fluid-structure system. The corneoscleral shell has been modeled as a thin and thick shell, taking into account material nonlinearities in the thick case. Numerical results obtained for the attached eye demonstrate a fair sensitivity of the resonance frequencies to the variations of the IOP; thus, founding the interest of the surveillance of the resonance frequency of the eye.     

Study of bone remodeling of two models of femoral cementless stems by means of DEXA and finite elements

Background  

A hip replacement with a cemented or cementless femoral stem produces an effect on the bone called adaptive remodelling, attributable to mechanical and biological factors. All of the cementless prostheses designs try to achieve an optimal load transfer in order to avoid stress-shielding, which produces an osteopenia.     

Elastic-plastic finite elements analysis of transient and residual stresses in ceramo-metal restorations

Transient and residual stresses occurring in partially fixed dental prostheses after the firing process can be calculated with elastic or elastic-plastic finite element analyses (FEA). In this study, firstly, the mechanical and thermal properties at various temperatures of the materials used in a porcelain fused metal (PFM) system were obtained by experimental and literature studies. The effects of viscoelastic and viscoplastic behaviours of the dental porcelain at the elevated temperatures were reflected onto its elastic properties. The equivalent heat transfer coefficients were determined experimentally by measuring temperatures and the results were supplied as input to the 3D finite elements analysis. It has been observed that the maximum stresses occur within a short time period after cooling begins and that stresses decrease during the cooling process and remain at a constant value at the end of cooling; these are the thermal residual stresses.     

Evolution of growth by genetic accommodation in Icelandic freshwater stickleback

Classical Darwinian adaptation to a change in environment can ensue when selection favours beneficial genetic variation. How plastic trait responses to new conditions affect this process depends on how plasticity reveals to selection the influence of genotype on phenotype. Genetic accommodation theory predicts that evolutionary rate may sharply increase when a new environment induces plastic responses and selects on sufficient genetic variation in those responses to produce an immediate evolutionary response, but natural examples are rare. In Iceland, marine threespine stickleback that have colonized freshwater habitats have evolved more rapid individual growth. Heritable variation in growth is greater for marine full-siblings reared at low versus high salinity, and genetic variation exists in plastic growth responses to low salinity. In fish from recently founded freshwater populations reared at low salinity, the plastic response was strongly correlated with growth. Plasticity and growth were not correlated in full-siblings reared at high salinity nor in marine fish at either salinity. In well-adapted lake populations, rapid growth evolved jointly with stronger plastic responses to low salinity and the persistence of strong plastic responses indicates that growth is not genetically assimilated. Thus, beneficial plastic growth responses to low salinity have both guided and evolved along with rapid growth as stickleback adapted to freshwater.     

Genotoxic effects of fluoride evaluated by sister-chromatid exchange

The purpose of this investigation was to study the genotoxic potential of fluoride (in the form of sodium fluoride, NaF) using in vitro and in vivo sister-chromatid exchange (SCE) assays with Chinese hamster cells. The NaF concentrations used in cultures of Chinese hamster ovary (CHO) cells ranged from 0 to 6.3 mM, both with and without S9 activation. Fluoride analysis of the culture medium demonstrated that it contained little indigenous fluoride, and the concentration of added fluoride was not affected by the components of the medium or the S9 mix. The CHO cells cultured in 6.3 mM NaF almost vanished, and at the concentration of 5.3 mM NaF in cultures without S9 microsome, only M1 cells were observed. In in vivo studies, Chinese hamsters were intubated with NaF dosages of 0, 0.1, 1.0, 10, 60 and 130 mg/kg, and the bone marrow (CHBM) cells were examined for SCE frequencies. Bone fluoride data showed that the intubated NaF was effectively absorbed. Death occurred in 3 of the 8 animals given 130 mg NaF/kg. The results indicated that NaF, in dosages up to 5.3 mM in CHO cell cultures and 130 mg/kg in in vivo CHBM cells, did not significantly increase the SCE frequencies over those observed in the negative (distilled water) controls. However, examination of the cell cycle revealed an inhibitory effect of NaF on cell proliferation with doses of NaF at or greater than 1.0 mM in cultured CHO cells and at or greater than 60 mg NaF/kg in in vivo CHMB cells. The results of the present study indicated an inhibition of the cell cycle and death of the cells with increasing concentrations of fluoride but not effect of fluoride on SCE frequency in CHO and CHBM cells.     

Tissues and organs as indicators of intestinal absorption of minerals and trace elements,evaluated in rats

Tissue and organ deposition and blood parameters were evaluated as indices of mineral and trace element absorption in rats. The absorption of elements was quantified in relation to nitrogen retention, i.e., considering the weight gain and new tissue synthesis. A rapeseed meal diet was supplied with three levels of calcium, two levels of zinc, and two levels of copper in a factorial design. In general, an increase in dietary mineral content increased the relative absorption, which in turn, increased the tissue deposition progressively. Striated muscle, however, did not respond to either an increased calcium or zinc supply. Furthermore, an increased calcium absorption caused a depression of the fractional phosphorus and magnesium content of femur bones. The copper content of the kidneys and the heart muscle was directly proportional to the amount of absorbed zinc and iron, respectively. The iron content of tissues was, in general, inversely proportional to zinc absorption and showed a tendency to be directly proportional to copper absorption. The zinc level in tissues was, in a similar way, inversely correlated to measured calcium absorption. In conclusion, interactions between elements do not only affect the intestinal element absorption, but also the distribution of already absorbed elements in tissues and organs.     

Genetic basis of adaptive evolution of a polyphenism by genetic accommodation

Polyphenisms are evolved adaptations in which a genome produces discrete alternative phenotypes in different environments. In this study, the genetic basis of the evolution of a polyphenism by genetic accommodation was investigated. A polyphenic strain and a monophenic strain of Manduca sexta (L.) were crossed and the F(1) offspring and backcross progeny were analysed. The larval colour polyphenism was found to be regulated by one sex-linked gene of major effect and many smaller effect modifier genes. The finding shows that the mechanism of genetic accommodation relies on genetic changes that are consistent with the current view of the genetic basis of adaptive evolution.     

On some distribution functions for non-linear switching elements with finite dead time

Summary The output distribution function of a non-linear switching element with a Poissonian sequence of impulses at the input is calculated by a straightforward method. The dead time of the element is taken into account. Some limiting properties of this distribution and its mean value are studied.The present research has been sponsored in part by the Air Force Avionics Lab., Research and Technology Div., Wright-Patterson Air Force, Air Force System Command — U.S.A.F. — Government of United States of America, Contract no. AF 33(615)-2786.