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An investigation is described in which the effect of both technical and physiological factors on muscle fibre size was examined. Size was determined by measuring the cross-diameter of 16,450 individual fibres by means of an ocular micrometer. In cases where an animal was represented by a single muscle sample, 100 fibres were measured, otherwise fifty measurements were recorded per sample. 2. The material consisted of muscle samples always taken from the same position along the length of the muscle and immediately afterwards fixed for varying periods of time in 10% formalin. Samples were obtained from various sources, principally from experiments conducted in the past at the School of Agriculture, Cambridge, but also from slaughterhouses and contemporary investigations. These latter samples were treated in the same manner to ensure consistency. 3. From the results of an investigation on the effect of formalin fixation on muscle fibre diameter, carried out on samples obtained from a mature buck rabbit, it was tentatively concluded that although shrinkage does occur the effect is only slight; the difference between measurements of fresh compared with fixed fibres being non-significant statistically. Shrinkage apparently ceased as soon as the fixative had penetrated the sample, since no further changes could be detected after prolonged fixation. 4. The same material was used to study the effect of heat. Application of heat (boiling) for a matter of seconds caused an immediate, highly significant, shrinkage.of individual fibres, but continued heattreatment resulted in no further changes. Though heat caused the fibre to lose its characteristic striated appearance, there were no signs of fibres disintegrating or of the sarcolemma becoming detached from its protoplasmic contents. 5. From the results of previous investigations it was deduced that the measurement of 100 fibres per sample provides a reliable mean value and a representative indication of the dispersion in a given muscle, or at a given age. Results obtained in the present study demonstrated a slight tendency for larger fibres to be measured as the period of observation proceeds, a factor which should be guarded against. 6. The effect of species on muscle fibre diameter was examined by comparing fibres of m. gastrocnemius of the rabbit, the pig, the sheep and cattle at birth and maturity. Only male animals were included in the comparison. It was shown that no relation exists between muscle fibre size and body size at either age. At birth the rabbit and sheep had rather similar-sized fibres, while those of the pig and cattle were respectively smaller and larger in diameter. At maturity the pig had the largest fibres, followed in decreasing order by the rabbit, cattle and sheep. The size of muscle fibres at maturity was accounted for by the degree of post-natal development in body weight of the particular species. 7. The effect of breed was studied on two groups of steers, the one consisting of Dairy Shorthorns and Dairy Shorthorn-crosses, and the other of Friesians and Friesian-crosses. Samples were available from m. longissimus dorsi for each of thirty-four animals: 100 fibres were measured per sample. The Friesians and their crosses had significantly thicker muscle fibres than the pure- and cross-bred Dairy Shorthorn steers. The largest differences existed between the respective pure-bred animals; differences between Friesian × Angus and Dairy Shorthorn × Angus, and Friesian × Hereford and Dairy Shorthorn × Hereford, though fairly distinct, were, however, not statistically significant. 8. The effect of age was investigated on a group of forty-one lambs of different nutritional status and sex, and ranging in age from birth to 290 days. Muscle fibre diameter was shown to increase in general with age, while a consistent decline in the coefficient of variation was regarded as indicative of the fact that muscular growth during post-natal life occurs essentially by hypertrophy of individual fibres, there being no increase in the number of fibres after birth. 9. Correlating changes in muscle fibre diameter with corresponding changes in weight, indicated that muscular growth is primarily a function of physiological age, and not strictly one of chronological age. Though highly significant correlations were established between mean fibre diameter and body and carcass weights, the strongest correlation was shown to exist between the former variate and muscle weight. However, a correlation of an even higher order was obtained between the square of muscle fibre diameter and muscle weight. It was attempted, by means of linear regressions, to indicate the contribution of length growth of the fibres to increments in muscle weight. The need for further investigation is, however, apparent; insufficient data in the present study made it impossible to elucidate this point altogether. The relationships were confirmed by an analysis of twenty lambs of the same breed, all slaughtered at 112 days of age; the heavier lambs had larger fibres than their lighter counterparts, very nearly proportional to differences in weight of muscle. 10. Sex differences in muscle fibre diameter could very nearly be accounted for on a basis of muscle weight alone at birth and at a carcass weight of 13·6 kg. At 290 days of age, high-plane wethers had but slightly thicker fibres than their female counterparts, despite a significantly heavier musculature. This was ascribed to differences in length of muscle (as shown by bone measurements), and also to differences in composition of the muscle. The results of chemical analyses were presented to prove that the muscles of wethers at that age contain greater amounts of intra-muscular (chemical) fat, hence the apparent increases in the weight of muscle could not be accounted for by an increase in the diameter of component fibres. 11. The effect of nutrition was studied in both lambs and mature ewes and shown to influence muscle fibre diameter appreciably at all ages. However, at birth the differences, though in favour of the high-plane lambs, were not significant statistically, probably due to small numbers in the respective groups. Muscle fibre diameter of mature ewes on a supermaintenance diet increased in proportion to increases in total muscle, while on a submaintenance diet the opposite effect was found. It appeared that continuation of the supermaintenance treatment would have resulted in but little additional changes in the diameter of fibres, while prolongation of the submaintenance treatment probably would have caused considerable further shrinkage of individual fibres. 12. The effect of the individual muscle was studied by comparing absolute and relative development of fibres of m. longissimus dorsi, m. rectus femoris and m. gastrocnemius. At birth m. gastrocnemius possessed the largest fibres and m. longissimus dorsi the smallest. On the whole, fibres of m. longissimus dorsi, the muscle being situated in a late maturing part of the body, showed greatest relative increases during post-natal life, while those of m. gastrocnemius, an earlier maturing muscle, increased latest. On an average, m. rectus femoris had larger fibres than m. longissimus dorsi at maturity, those of m. gastrocnemius being the smallest in absolute measure. Comparing the relative degree to which different muscles develop under high and low planes of nutrition, muscle fibre measurements indicated that the low-plane animals at a chronological age of 290 days, resembled their 60-day-old high-plane counterparts in anatomical development. In mature animals, the early maturing gastrocnemius appeared to benefit most initially from a supermaintenance diet; however, during the later stages of the experiment, m. longissimus dorsi fibres showed the greatest relative increases, followed in order by m. rectus femoris and m. gastrocnemius. On the submaintenance ration, m. longissimus dorsi fibres appeared to be reduced in size at a greater rate initially than those of the other muscles. Individual variation, however, made it extremely difficult to generalize during subsequent stages. 13. From width and depth measurements on m. longissimus dorsi (or ‘eye muscle’) recorded at the junction of the thoracic and lumbar vertebrae, it was shown that width is the earlier maturing dimension and less affected by nutritional factors than depth. Though significant relationships were found between mean fibre diameter and both muscle width and depth, the latter dimension was the more strongly correlated with changes in thickness of fibres. 14. It has been suggested that work of this nature might provide a suitable basis for estimating the amount of muscular tissue in a carcass; such a relationship would be of great assistance to students of meat physiology who have to resort to laborious dissection techniques for data on carcass composition. It is obvious, however, that factors such as species, breed, and possibly sex, would have to be considered in an attempt to establish a relationship of this kind. Furthermore, the effect of the individual muscle used for test would demand consideration, different muscles being influenced to varying degrees by age and nutrition. A late maturing muscle probably would furnish the most reliable criterion; in the light of evidence produced by this study, m. longissimus dorsi sampled at the junction of the lumbar and thoracic regions has been suggested as most suitable.
D. M. Joubert (Wed,) studied this question.