An experimental study of muscle growth in the rabbit.

Descriptions of the growth of voluntary muscle are based almost entirely on the examination of histological sections. The greater part of a growing muscle fibre has the characteristic striated structure, but at its extremities there is a region of undifferentiated sarcoplasm where rapid nuclear proliferation occurs. This is presumably accompanied by formation of new sarcoplasm which later becomes differentiated to form the structure of the striated muscle fibre. Longitudinal growth is thus generally believed to take place mainly, if not exclusively, at the end of the fibre ; and such a view has been supported by Riedel (1874), Froriep (1878), Felix (1889), Mingazzini (1889), Morpurgo (1899), Heidenhain (1911), Schmidt (1927) and Carr (1931). Since in a strap muscle or a fusiform muscle many fibres terminate at the ends of the muscle belly, it might be supposed that the growth of the muscle as a whole would occur mainly here. This supposition has not been proved, however, and there is in fact very little direct evidence concerning longitudinal growth of the muscle belly. Perhaps the most satisfactory is that of Speidel (1938), who, after prolonged observation of the myomeres in the tails of living tadpoles, concluded that growth actually does take place in the way described. In a study on tendon growth Crawford (1950) described a method of placing small indian ink marks at intervals along the tendons in young rabbits. The distance between the marks was measured during the operation and again after an interval, and the site of growth was thus determined. This method has now been applied to the muscle belly, where it proved equally informative. An additional method was devised for marking the deep part of the muscle substance by inserting at intervals thin pieces of wire into the muscle belly and measuring the distance between them on radiographs taken before and after a period of growth. These experiments provided confirmation of the results obtained by the indian ink technique. It is widely believed that the form of a muscle belly is highly adapted to its function; this has been stressed, for example, by Frohse and Fr#{228}nkel (1908). Haines (1932) suggested that a muscle belly usually contracts only by a certain proportion of the length of its fibres, and that the length of the muscle belly is correlated with the distance it is required to contract to cause full movement of the joint on which it acts. It may be possible to deduce how the length of the muscle belly is adapted to its function, but the immediate factors that determine the extent of its growth before and after birth are quite unknown. An experiment was therefore devised to alter the distance through which a muscle had to contract in order to produce its normal degree of movement, with the intention of determining whether this leads to any alteration in the extent of its ultimate growth.