Sections, making transverse

Fig. 15.2 Diagram showing a transverse cross-section of a cerebral capillary. The endothelial cells, responsible for the main barrier properties of the blood-brain barrier are separated from the astrocyte foot processes, pericytes and occasional neurons by the basement membrane. All these components make up the blood-brain barrier.

Although nearly all creep and stress-relaxation tests are made in uniaxial tension, it is possible to make biaxial tests in which two stresses are applied at 90° to one another, as discussed in Section VI. In a uniaxial test there is a contraction in the transverse direction, but in a biaxial test the transverse contraction is reduced or even prevented. As a result, biaxial creep is less than uniaxial creep--in cquihiaxial loading it is roughly hall as much for equivalent loading conditions. In the linear region the biaxial strain 2 in each direction is (255.256)... 

Profiles are all extruded articles having a cross-sectional shape that differs from that of a circle, an annulus, or a very wide and thin rectangle (flat film or sheet). The cross-sectional shapes are usually complex, which, in terms of solving the flow problem in profile dies, means complex boundary conditions. Furthermore, profile dies are of nonuniform thickness, raising the possibility of transverse pressure drops and velocity components, and making the prediction of extrudate swelling for viscoelastic fluids very difficult. For these reasons, profile dies are built today on a trial-and-error basis, and final product shape is achieved with sizing devices that act on the extrudate after it leaves the profile die. 

In a first approach to the study of beam bending, it is convenient to make some hypotheses (1). The first of these hypotheses is that the sections that are flat before flexion remain flat after flexion. For slender beams—that is, for beams whose transverse dimensions are small in comparison with their length—this hypothesis is substantially correct. In this case, the shear effects in the cross sections are relatively negUgible. It will be further assumed that the inertial forces arising from the rotation of each element around its center of mass can be ignored. This is, in fact, the second hypothesis. 

The spatial resolution of FT-IR microspectroscopy, without sacrificing spectral quality and resolution, makes imaging possible. Shortly after the introduction of the first research-quality IR microscope by Messerschmidt and Sting in 1986, Wetzel, Messerschmidt and Fulcher reported spectra obtained from wheat kernel transverse sections in situ, and compared them with flour milling fractions [7]. This was achieved with an accessory IR-PLAN microscope optically interfaced to a Nicolet interferometer bench. Subsequently, at the Agriculture Canada laboratory the same model IR-PLAN was interfaced to a Bomen Michelson IR 100 spectrometer such that, over the period of a year, transverse sections of wheat kernels, vanilla beans, peppercorns and soybeans were manually line-mapped to reveal any differences in microchemical structural characteristics between their different botanical parts [8]. 

It turns out that the secular part depends on the spectral density at zero frequency, 7(0). We can see that this makes sense as this part of transverse relaxation requires no transitions, just a field to cause a local variation in the magnetic field. Looking at the result from section 8.5.2 we see that 7(0) = 2tc, and so as the correlation time gets longer and longer, so too does the relaxation rate constant. Thus large molecules in the slow motion limit are characterised by very rapid transverse relaxation this is in contrast to longitudinal relaxation is most rapid for a particular value of the correlation time. 

What fraction of the total volume of the fibril is occupied by the filaments is a controversial matter. Apart from the uncertainty as to how much of the transverse spacing of 115 A. is taken up by the filament and how much is interstitial space, there is no agreement as to the extent to which the fibril is filled by bundles of filaments at all. Draper and Hodge (1949) regard the fibril as a sort of tube of which only a thin wall contains the axially aligned filaments (c/. Pease and Baker, 1949). This view is based on the excellent electron optical transparency, which makes it possible to distinguish each individual filament even in relatively broad fibrils. Other authors assume either definitely (Rozsa et al., 1950) or tentatively (Hall et al., 1946 Hoffmann-Berling and Kausche, 1950) that the whole cross-section is occupied by filaments. 

Touring the past decade a number of research laboratories have devel-oped procedures for the rapid synthesis of amino acids labeled with short-lived, positron-emitting radionuclides. These tracers make possible the study of regional amino-acid metabolism in the living organism by external detection of the y photons produced by positron annihilation. This decay mode also permits the determination of label distribution in transverse sections through the body by means of positron emission... 

---