Productos finales de la digestión anaerobia

apparently haphazard distribution. One mosaic is picked out by circles in each of the three boxes.

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The typing system thus based on the lamination patterns and on these additional characteristics could then be used to assign almost every well-labelled large ganglion cell in the plots to a particular cell

type. Three independent large cell populations could thus be identified and made available for various morphometric and population analyses

(Fig. 6).

Digitising the cells: A4 acetate sheets were attached to the grid sheets or photographs, and the locations of the defined cells were marked on them using specific colour codes for different types of cell. Each grid sheet or photograph was coded by row and column according to its position in the specimen, and the cell coordinates were digitised, sheet by sheet, on a high-resolution graphics tablet (Summagraphics Summasketch II Plus) linked to a microcomputer. For every cell in turn, the retina-wide coordinates were then calculated automatically from its cursor coordinates on the tablet, the base coordinates on the sheet, and the sheet code denoting its row and column within the total array (Cook,

1987). The results were stored in a disc file.

2:2.3 Nearest-neighbour distance analysis

The following computations and comparisons were made with the help of special-purpose computer programs (Cook and Becker, 1991)• The nearest neighbour distance (NND) for each cell in the mosaic, the frequency distributions of these distances and their means and standard deviations were computed. Kolmogorov-Smirnov one-sample comparisons^ were made between these distributions and two theoretical ones: the Gaussian with the same mean and standard deviation and the Rayleigh distribution, a distribution of the Poisson type for a random population of the same average density (Wassle and Riemann, 1978; Rodieck, 1991)*

The mosaic ratio (the ratio of the mean NND to the standard deviation: Schall et al., I987) and the dispersion index (the ratio of the observed mean NND to that expected for a random population: Clark and Evans, 1954) were also calculated. The mosaic ratio for a non-

1 Th e K o l m o g o r o v - S m i r n o v on e -s a mp l e test is a test of g o o d n e s s - o f - f i t . It a s s e s s e s the d eg r e e of a g r e e m e n t b e t w e e n the di st r i b u t i o n of a set of sample va l u e s ( o b s e r v e d sc o res ) a n d so me s p e c i f i e d t h e o r e t i c a l di st r ib ut i on . The result in di cat es wh e t h e r the sa mp l e s co r es ca n r e a s o n a b l y be t ho ugh t to h a v e com e fr om a po p ul a t i o n wit h the theor eti cal d i s t r i b u t i o n (Siegel, I956).

random distribution has no theoretical limits but in practice lies between 2.5 and 6.0 for most documented ganglion cell mosaics. The dispersion index takes a value close to unity when the underlying

distribution is random and has a theoretic maximum value of 2.149 for a completely regular hexagonal array {Clark and Evans, 1954) but in

practice lies between 1.2 and 1.6 for most ganglion cell mosaics.

The nearest-neighbour analysis of a mosaic distribution calls for good dendritic labelling of a high proportion of the cell population in a substantial part of the retina. Some of the retinae examined fell short of fulfilling these criteria. Techniques like "scratch-labelling"

(R.V. Stirling, personal communication) and labelling from a partial nerve cut were responsible for this in some cases. In others, uneven processing or damage during dissection was the cause. However, quite well-labelled, isolated cells or small patches of cells were found in these retina. The large ganglion cells in such retinae could be assigned to different types by assessing the major type-specific

qualitative features of the individual cells. Some of these cells were drawn or photographed for studies like assessment of dendritic

interaction.

2:2.4 Spatial correlograms

The spatial relationships between the cells were also expressed through spatial correlograms and density recovery profiles, and effective radii of exclusion were calculated, using computer programmes written by J.E. Cook following procedures devised by Rodieck (1991)' These terms

warrant some explanation.

To create spatial auto-correlograms, the spatial coordinates of the cells, as entered into the computer from the grid sheets, were retrieved from disc. Each cell was treated in turn as a 'reference cell' and placed at the centre of the correlogram area. Then all its neighbours within a defined radius (usually 500 pm) were plotted round it. To visualise the process and the effect, we can imagine, as Rodieck described, marking a 'cell' on a plot with a plus sign. A reference point is placed near the middle of a translucent paper. The paper is then placed on the plot, the reference point is aligned with the plus sign, and the positions of all the other cells on the plot that lie

Figure 7