Structure of Muscle

Involuntary or Smooth Muscle.

This particular form of muscle tissue when separated into its single constituents is seen to consist of fibres possessing a typical long spindle shape. The central part is somewhat swollen and contains an elongated nucleus centrally placed. The ends of the fibres are drawn out and pointed sharply. There is no definite surrounding membrane to each cell. In most of the cells, especially the larger, a distinct longitudinal marking can be seen. This is due to the presence of the fibrils which run the length of the fibre and are probably the essential contractile elements.

In most instances the cells are arranged to form bundles or sheets of contractile substance. In each bundle or sheet the cells are cemented to one another so that they may all act in unison. The cementing material is apparently membranous and is so arranged that contiguous fibres are only separated by a single layer of membrane. According to some, neighbouring fibres are connected to one another by minute offshoots, and these commu nications serve to explain the manner in which the contraction is observed to pass from fibre to fibre along a sheet composed of the muscles.

Involuntary muscle is the variety of muscle tissue found in the walls of the hollow viscera, such as the stomach, intestines, ureter, bladder, uterus, etc., and of the respiratory passages, in the middle coats of arteries, in the skin and the muscular trabeculae of the spleen. The arrangement is very typical, for instance, in the small intestine. Here the muscular coat consists of two layers of muscle. Each is in the form of a sheet which varies greatly in thickness in different animals. In the inner sheet the fibres, all

parallel to one another, are disposed with their long axis transverse to the direction of the gut. In the outer layer, the direction of the fibres is at right angles to this. In a viscus with thick muscle walls the fibres are bound into bundles and the bundles may run in all directions. In some instances the bundles may form branching systems, thus constituting a network, notably in the bladder. In other instances, e.g., the villi of the small intestine, the muscle fibres are separate, forming a felt-work with wide meshes.

Heart Muscle.

The fibres of which the muscular walls of the heart are composed though cross-striated are not voluntary. Each fibre is an oblongated cell possessing distinct transverse and less distinct longitudinal striations. There is no sarcolemma, and the nucleus of each fibre is placed in the centre. The longitudinal striation is due to the presence of fibrillae, each of which is cross striated. These lie parallel to one another in the cell, the sarco plasm surrounding them being much more abundant in these fibres than in striated muscle. The fibrillae are arranged in rows, and when a transverse section of one of these fibres is examined it is seen that the rows radiate away from the centre of the cell.

A further distinctive character of cardiac muscle fibres is that they frequently branch, the branches uniting with others from neighbouring cells. Moreover, the ends of the fibres are attached to corresponding faces of other cells, and through these attached faces the fibrillae pass, so that there is an approximation to the formation of a syncytium. (T. G. B.)