Attention is directed to the paucity of information regarding the
morphological growth and development of muscle, particularly in connection with the microscopical elements comprising muscle. Such knowledge is of value, not only in affording a basis for studying meat quality in different species of domestic animals, but also in understanding the principles underlying the function of different muscles.
In a preliminary study, statistical methods were employed in order to
ascertain suitability of sampling and measuring muscle bundles and muscle fibres. For sampling the bundle length of a muscle, a selection of ten measurements gives a sufficiently reliable mean. For sampling the diameter of muscle fibres, a selection of one hundred measurements affords a sufficiently reliable mean. Variable results are obtained when a measure of fibre diameter is calculated from cross-sections of muscle, probably due to the difficulty of cutting sections at right angles to the line of the fibres.
Measurement of the cross-diameter of short lengths of muscle fibres yields more reliable results, and the values obtained are greater than those obtained by measuring fibre diameter in cross-sections. For calculating the texture of a muscle, a selection of twenty bundles provides a fair estimate of the number of fibres comprising the individual muscle bundle.
With the object of establishing the general principles of morphological development during post-natal life, the relative changes were studied in the tissues and anatomical units of M. Gastrocnemius medialis and M. Psoas major, in a series of male rabbits killed at intervals from birth to fourteen months of age. Throughout this study, the quantitative data were subjected to statistical analysis. Qualitative changes were not considered.
The work is to be regarded as a preliminary investigation, with the purpose of drawing attention to the main principles involved in the growth of the muscles studied.
Both muscles undergo extensive enlargement during the growth of the rabbit. Relative to the Psoas muscle, M. Gastrocnemius makes a greater proportion of its growth in mass early in life.
Although no important differences were revealed in the mechanism of
lengthening of these muscles, there is a striking difference in the manner whereby the individual muscle bundles contribute to this length increase.
Whereas the Psoas muscle lengthens by virtue of a persistent increase in the length of its component bundles, Gastrocnemius bundles do not
lengthen after the first two months of life. During the remainder of the
lifetime of the rabbit, they do not contribute to the appreciable degree of lengthening which is still manifested by the Gastrocnemius muscle.
Because of the oblique position of the bundles, thickening of these bundles appears to be the principal factor promoting this increase in length of the muscle. A change in the relative position of these bundles within the Gastrocnemius muscle tends to increase the depth of the muscle.
Both muscles vary appreciably in form. M. Gastrocnemius is short,
with a pronounced belly, and is more or less uniformly deep. M. Psoas is long, with a less marked belly formation, and becomes progressively thicker along its length from origin to insertion. In the absence of information regarding the working of these muscles, the advantages of the respective variations in form in promoting muscular efficiency cannot be discussed.
Contrary to expectation, the muscles do not widen or deepen during a
later stage of the lifetime of the animal than they increase in length.
Within M. Gastrocnemius, the individual bundles show well-defined
differential length relationships. In the young rabbit, the bundles
show a progressive increase in length along the length of the muscle from its origin to its insertion. In the older rabbit, the bundles are shorter a little distance beyond the muscle origin than at the origin itself, then exhibit a similar increase in length along the muscle to its insertion. The change from one system to the other occurs between 320-480 gm. live-weight, at about 3-4 weeks of age.
In both muscles the bundles continue to thicken throughout the period
of growth. Although there is only a slight difference in bundle thickness in the new-born rabbit, Gastrocnemius bundles subsequently thicken at a greater rate and become increasingly thicker than the Psoas bundles, as the animal becomes older. Hence M. Gastrocnemius assumes a coarse texture relative to the Psoas muscle.
The bundles comprising the Psoas muscle contain a larger number of
individual fibres than the Gastrocnemius bundles. However, as regards muscle texture, this numerical superiority is more than offset by the greater fineness of the fibre in the Psoas bundles.
No evidence was obtained of any decrease in the number of muscle
fibres during the post-natal life. Hence, the enlargement in form and the increasing weight of the muscles must be considered to be due mainly to an increase in the size of the existing muscle fibres.
Length of Gastrocnemius muscle fibre is an extremely early developing character. In the Psoas muscle the fibres continue to lengthen throughout the period of growth. This increase in the length of Psoas fibres is largely responsible for the increase in bulk of the muscle. Within each muscle, the fibres show a well-defined thickness relationship.
In M. Gastrocnemius, the fibres at birth are thinner in mid-muscle
than at both ends of the muscle. At all succeeding stages the Gastrocnemius fibres are thinnest near the origin, then become progressively thicker along the muscle to a point near the muscle insertion. By contrast, the Psoas fibres are at all stages thinnest in the middle of the muscle. Presumably these differences in the relative size of the contractile units are dictated by functional considerations.
The Gastrocnemius fibres thicken to a greater degree and become much thicker than the fibres in M. Psoas. Because of the early cessation of length growth in the Gastrocnemius muscle fibre, this thickening of the component fibres largely accounts for the increase in the bulk of M. Gastrocnemius.
Any application of the data to a different species of animal, or to
different muscles, will naturally require caution. However, the general
principles for the two basically different classes of muscle studied are
probably similar in allied types of a wide variety of muscle. The essential structural difference of these two types is the direction of the muscle fibres.
In M. Gastrocnemius, of pinnate structure, the short muscle fibres join the tendon at an acute angle, whereas in the Psoas muscle, the fibres are characterised by their parallel arrangement from end to end of the muscle.
For a large bulk of muscle, the general conclusions may be of value in
providing a basis for further study of muscle belonging to either of these types.
No mention has been made whether the data collected for the various
measurements obey Huxley's allometric law. This aspect of the work is undergoing investigation, and will be presented in a future publication. At this stage, it can be stated that a straight-line relationship in logs. can be
fitted to all the data by the method of least squares (i.e. allometric growth),
excepting Gastrocnemius weight, Gastrocnemius depth, and muscle length
and bundle length for both muscles. However, graphical analysis by
means o£ confidence-regions eliminates in addition a number of the measurements
showing a straight-line relationship in logs. by the least squares
method. Thus, the method of curve-fitting completely satisfying the confidence
region criterion shows that M. Gastrocnemius grows in allometric
manner only for fibre diameter. Similar treatment for the Psoas muscle
leaves only weight, width, and depth of muscle in the category of measurements
which obey Huxley's law. Until it is possible to elaborate the reasons
for the discrepancies, as well as the dissimilarities in the muscles studied,
no useful purpose can be served by discussion or comparison.
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