When changes take place in protoplasm adapting it specially for contraction, it is termed muscle tissue. The large masses of this tissue attached to the skeleton so as to move its various parts, form the flesh of the higher animals. Muscle tissue is, almost invariably, connected with nerve tissue, and acts in response to stimuli communicated from the nerves. In some of the lower animals, the two tissues are so intimately related that it is not easy to distinguish them, and the development of both progresses equally as we ascend the scale of animal life. They are nearly related in their origin, or even spring from the same primitive tissue. In fact, as has already been mentioned {vide p. 46), they form but one structure in some of the more simple and less differentiated animals. The neuro-muscular tissue, which is formed from the outer layer of the embryo, is the forerunner of the muscles as well as of the nerves of the embryo of the higher animals.

In the higher animals and man muscle tissue consists of two distinct kinds of textures, known as -

{a) Smooth, or non-striated muscle.

(b) Striated muscle.

In the smooth muscle the individual elements present the characters of an elongated and flattened cell, and contain a single long nucleus. They contract very slowly, and require a comparatively long time for the nerve influence to affect them, so that an obvious interval exists between the moment of their stimulation and their contraction. They are found in the internal organs and in situations where gradual and lasting contractions are required. They receive their nervous supply generally from the sympathetic system, and perform their duty without our being conscious of their activity or being able to control it by our will.

Striated muscle tissue is made up of cylindrical fibres of such length that both extremities cannot be brought into the field of the microscope at the same time. Their exact relation to cells is not so easily made out as in smooth muscle, and doubtless varies in different muscles. Sometimes the fibres are made up of single cells, and in other cases they are formed by the permanent fusion of several cell elements which never differentiate into separate elements, owing to the imperfect division of the cells, but make up one mass, the multiple nuclei of which alone make its mode of origin apparent. The contractile substance is made up of two kinds of material, one of which refracts light singly, while the other is doubly refracting. These are ranged alternately across the fibre, making the transverse markings or striae from which it gets its name. This striated material is quite soft and is encased in a thin homogeneous elastic sheath called sarcolemma, which fits closely around the soft contractile substance.

Cells of smooth muscle tissue from the intestinal tract of rabbit.

Fig. 22. Cells of smooth muscle tissue from the intestinal tract of rabbit. (Ranvier.) A and B. - Muscle cells in which differentiation of the protoplasm can be well seen. (Schafer).

Two fibres of striated muscle, in which the contractile substance.

Fig. 23. Two fibres of striated muscle, in which the contractile substance (m) has been ruptured and separated from the sarcolemma (a) and (s); (p) space under sarcolemma. (Ranvier).

This form of muscle is the widest departure from the primitive protoplasmic type, being specially modified so as to perform strong and quick contractions. It moves with wonderful rapidity, contracting almost the instant its nerve is stimulated. It forms the great mass of the quick-acting skeletal muscles, being attached to the bones by bands composed of a form of fibrous tissue, which form the tendons and fasciae. Muscles made of striated tissue are commonly under the control of the will, and hence are frequently spoken of as voluntary muscles, but this term is misleading, for many striated muscles are not governed by voluntary control.

The Connective Tissue group, coming exclusively from the mesoblast, exhibits very great varieties of form. Its cells differ much from the epithelial cells both in their character and their relations, and particularly in the adult tissues.

Under the heading Connective Tissues are generally classed all those which support the frame and hold together the various other tissues and organs. They are -

1. Mucous and retiform connective tissues.

2. White and yellow fibrous tissue.

3. Cartilage.

4. Bone.

5. Endothelium.

The cells of all these tissues have the property of manufacturing some material which does not generally enclose them as a cell wall, but remains between the cells and forms the intercellular substance. The younger the tissue the greater is the proportion of its cellular constituents, and the older the tissue the greater will be found the preponderance of the intercellular substance.

Transverse section of the chorda dorsalis and neighboring substance.

Fig. 24. Transverse section of the chorda dorsalis and neighboring substance. a, cartilage cells; b, cell of the middle layer of embryo; c, mucous tissue; d, boundary of chorda. (Cadiat).

Cells of mucous tissue with branching processes (B) and a couple of elastic fibres (F).

Fig. 25. Cells of mucous tissue with branching processes (B) and a couple of elastic fibres (F).

(Ranvier).