Filament lengths in resting and excited frog muscles have been measured in the electron microscope, and investigations made of the changes in length that are found under different conditions, to distinguish between those changes which arise during preparation and the actual differences in the living muscles. It is concluded that all the measured differences in filament length are caused by the preparative procedures in ways that can be simply accounted for, and that the filament lengths are the same in both resting and excited muscles at all sarcomere lengths greater than 2

The organisation of the myofibrils and the sarcoplasmic reticulum in frog slow muscle fibres has bccn compared with that in twitch fibres. It has been found that the filaments have the same length in the two types of fibre, but that there arc differences in thcir packing: (a) in contrast to the rcgular arrangement of the I filaments near the Z line in twitch fibres, those in slow fibres are irregularly packed right up to their insertion into the Z line; (b) the Z line itself shows no ordered structure in slow fibres; (c) the fine cross-links seen betwecn thc A filaments at the M line level in twitch fibres are not present in slow fibres. The sarcoplasmic reticulum in slow fibres consists of two separate networks of tubules. One set of tubules (diameter about 500 to 800 A) is oricnted mainly in a longitudinal direction. The tubules of thc other network (diameter about 300 A) are oriented either transversely at approximately Z line level or longitudinally, connecting the transverse tubules. Triads are very rarcly found, occurring at only every 5th or 6th Z line of each fibril. The central clcment of these triads is continuous with the thin tubules. Slow fibres from muscles soaked in ferritin-containing solutions contain ferritin particles in the network of thin tubules, the rest of the sarcoplasm remaining free of ferritin.

No abstract

It has been known for many years that, when muscles are placed in hypertonic solutions, their loss in weight is generally less than would be expected if the muscle were a simple osmotic sac. Even in the strongest solutions muscles lose only about 20 % of their initial weight (see Loeb, 1897;Cooke, 1898). Overton (1902) showed that even when allowance was made for dry matter and for an extracellular space of about 20 % (corrections not adequately appreciated by his predecessors), there was still a discrepancy between the calculated and the measured changes in the weight of the muscle. He accounted for this by proposing that about a fifth of the water was bound within the cell in the form of 'Quellungswasser', which could not be displaced except by very large gradients of vapour pressure (see his pp. 139-142, 155). Overton's conclusion was disputed by Hill (1930), who showed that almost all the fibre water could dissolve solutes in a normal manner. The assumption that all the water in muscle fibres participates in osmotic effects was later shown to lead to a satisfactory explanation of the weight changes in mildly hypotonic and hypertonic solutions . It seemed to us that the discrepancy in strongly hypertonic solutions might be explained by Overton's own qualitative observation, that in such solutions the extracellular space appeared to be increased.In this paper we have confirmed and extended this observation of Overton's by actual measurement of the extracellular (sucrose) space in solutions with various osmotic strengths. This permits, by subtraction, a direct calculation of the amount of fibre water as a function of osmotic strength. It then turns out that in solutions up to four or five times as concentrated as normal Ringer's solution the muscle fibres behave as simple semipermeable bags containing a fixed amount of solute: there is no need to invoke binding of any appreciable part of the fibre water.

SUMMARY1. The fine structure of tortoise skeletal muscle has been studied with the electron microscope.2. The packing of the A filaments (1 6 jIt long) and the I filaments (2-35 ,t long) within the myofibril is similar to that in frog twitch fibres.3. The position of the N line within the I band is found to vary with the sarcomere length.4. The T-system tubules in these muscles form a three-dimensional network across the fibre at the level of the I band. On either side of the Z line it forms triad relationships with the other network of tubules of the sarcoplasmic reticulum.