In the bodies of animals we find the protoplasmic masses or cells of which they are composed variously modified, in order to perform special functions.

In some the power of nutrition is chiefly developed : and this we find in glands. In others the power of contractility is developed: and this we find in muscles, striated and non-striated.

In the course of special development towards the fulfilment of a particular function, the protoplasm of the muscular cells undergoes marked changes. But it must always be borne in mind that the protoplasmic elements of the body, however different from one another, always tend more or less to retain all the functions which are seen in an organism consisting of a single cell, a reference to which may sometimes throw much light upon the mode of life of the more highly organised tissues.

In amoebae or leucocytes the protoplasm contracts in any direction and when strongly contracted in tetanus they become spherical.

In muscle the protoplasm is specially modified and contracts chiefly in one direction, viz. that of its length, and, indeed, it is usually assumed that muscular fibre, either voluntary or involuntary, contracts in the direction of its length only.

But the probability of its contraction in a transverse direction also is to be borne in mind, and there are some phenomena which it is very hard to explain except on the supposition that muscle contracts transversely as well as longitudinally.1

We distinguish in muscle its elasticity, a physical property; and its contractility, a vital property.

1 Thus Weber found that when a muscle is loaded with a weight too great for it to lift, instead of shortening, it elongates. The usual explanation of this is that the elasticity of the muscle then becomes diminished; but according to Wundt the elasticity is not changed. If we suppose that stimulation tends to make the muscle contract transversely as well as longitudinally, the explanation is easy, for in this case, longitudinal contraction being prevented, the transverse contraction tends to elongate the muscle.

The word elasticity is applied to the tendency of the body both to resist change of its form, and to regain it when this change has been effected : so that ivory may be taken as the type of a very strongly elastic body. Indiarubber, on the other hand, is regarded as a feebly elastic body, because it does not strongly resist changes of form, although it tends very strongly to regain its original form after such changes. It is, however, popularly regarded as the perfect type of an elastic body. In talking of the elasticity of muscle, confusion is apt to occur; it is better, then, to avoid the term elasticity and to use the words suggested by Marey - extensibility and retractility. The extensibility of muscle is of two kinds - immediate and supplementary. When a weight is attached to it, it extends considerably; this is its immediate extensibility; it then goes on slowly and gradually lengthening for a considerable time, and this is supplementary extensibility. When the weight is removed the retractile power of the muscle again becomes evident, and there is immediate retractility and supplementary retractility, the muscle at once contracting to a considerable extent, and then continuing to do so slowly and gradually for some time afterwards.

The extensibility of a muscle is increased by stimulation, so that if a weight be hung on a muscle while it is contracted in consequence of stimulation, it will produce a greater extension than it would if applied to the same muscle in a state of rest; and if a muscle be loaded with a weight too great for it to raise, stimulation, instead of causing contraction, causes elongation.1 Heat renders the muscle less extensible and more retractile; cold has an opposite effect, rendering it more extensible and less retractile. Section of the nerve has a similar effect to that of cold. Fatigue increases the extensibility. Alkalis (potash or soda), in very dilute solutions, diminish extensibility; dilute acids (lactic acid) increase it. By the alternate application of alkalis and acids the muscle may be made to yield curves which, when recorded on a very slowly-revolving cylinder, are similar in form to the normal contraction curve recorded on a rapidly-revolving cylinder.1 Fig. 34.

Fig. 34.   Shows the action on muscle of caustic soda, 1 in 2,500, once renewed in 25 minutes, followed by the action of lactic acid, 1 in 500, once renewed in 25 minutes.

Fig. 34. - Shows the action on muscle of caustic soda, 1 in 2,500, once renewed in 25 minutes, followed by the action of lactic acid, 1 in 500, once renewed in 25 minutes. (Brunton and Cash.)

Fig. 35.   Shows the action on muscle of caustic potash, 1 in 2,500, twice renewed for 13 minutes, succeeded by the action of lactic acid, 1 in 500, for 18 minutes, and this by the action of caustic potash for 17.

Fig. 35. - Shows the action on muscle of caustic potash, 1 in 2,500, twice renewed for 13 minutes, succeeded by the action of lactic acid, 1 in 500, for 18 minutes, and this by the action of caustic potash for 17.5 minutes. (Cf. Fig. 50, p. 132.) (Brunton and Cash.)

1 Vide footnote, p. 117.

Fig. 36.   Shows the action of caustic potash, 1 in 1,500, on muscle for 18 minutes, succeeded by the action of lactic acid for 24 minutes. 1 is the contraction of normal muscle; 2, 3, 4, contractions of alkali muscle; 5, 6, 7, contractions of acid muscle on stimulation.

Fig. 36. - Shows the action of caustic potash, 1 in 1,500, on muscle for 18 minutes, succeeded by the action of lactic acid for 24 minutes. 1 is the contraction of normal muscle; 2, 3, 4, contractions of alkali-muscle; 5, 6, 7, contractions of acid-muscle on stimulation. (Brunton and Cash.)