presented by ordinary striated muscle. But it may further be noticed that the dim bands exert a powerful depolarising influence on polarised light. Hence when a piece of muscle is placed in the field of a polarising microscope and the prisms are crossed so that the field is dark, these bands appear bright. The granular lines have a similar but very much less marked effect.

36. As in the case of the preceding tissues so in that of muscle, the place of the adult tissue is occupied in the embryo by a mass of closely applied, undifferentiated nucleated cells. As development proceeds, some of these cells are converted into the tissues of the perimysium, but others increasing largely in size gradually elongate and take on the form of more or less spindle-shaped rods or fibres. Meanwhile the nucleus of each cell repeatedly divides, and thus each rod becomes provided with many nuclei, so that each fibre is really a multi-nucleate cell. Along with these changes the protoplasmic substance of the original cell becomes, for the most part, converted into the characteristically striated muscle substance, only a little remaining unaltered around each nucleus as a muscle corpuscle.

37. The many-nucleated cell metamorphosed into a muscular fibre is nourished by the fluid exuded from the adjacent capillaries, and it may be said to respire, insomuch as its substance undergoes slow oxidation at the expense of the oxygen contained in that fluid, and gives off carbonic acid. It is, in fact, like the other elements of the tissues, an organism of a peculiar kind, having its life in itself, but dependent for the permanent maintenance of that life upon the condition of being associated with other such elementary organisms, through the intermediation of which its temperature and its supply of nourishment are maintained.

The special property of a living muscular fibre, that which gives it its physiological importance, is its peculiar contractility. The body of a colourless blood corpuscle, as we have seen, is eminently contractile, insomuch as it undergoes incessant changes of form. But these changes take place at all points of its surface, and have no definite relation to the diameter of the corpuscle, while the contractility of the muscular fibre is manifested by a

A A

diminution in the length and a corresponding increase in the thickness of the fibre. Moreover, under ordinary circumstances, the change of form is effected very rapidly, and only in consequence of the application of a stimulus. When a contracting striated fibre is observed under the microscope all the bands become broader (across the fibre) and shorter (along the fibre) and thus more closely approximated. Some observers think that the clear bands are diminished in total bulk relatively to the dim bands; but this is disputed by others. When the fibre relaxes again the bands return to their previous condition. 38. Non-striated muscle.-This kind of muscle (also called plain or smooth muscle) which occurs in the walls of the alimentary canal, the blood-vessels, the bladder, and other organs, resembles striated muscle in being composed of fibres, which are bound together by connective tissue carrying blood-vessels and nerves; but the

12

FIG. 108.-A FIBRE-CELL FROM THE PLAIN, NON-STRIATED MUSCULAR COAT OF THE INTESTINE.

, granular protoplasm around the nucleus.

non-striated muscular fibre differs greatly from the striated' fibre. It is very much smaller, being only about 6μ in width, and from 20μ to 50μ in length, and therefore cannot be seen by the unassisted eye, whereas a large unbroken striated fibre is visible to a sharp eye. It has only one nucleus, possesses no sarcolemma, and its substance is not transversely striated. It is, in fact, a cell which has become elongated into a flattened spindle, with an oval or sometimes rod-shaped nucleus in its middle (Fig. 108). A number of such fibre-cells are united together by a minute quantity of cement or intercellular substance into a thin flat band, and a number of such bands are bound together by connective tissue into larger bands or bundles. Each fibre is capable of contracting, of shortening into a thicker oval.

39. Cardiac muscular tissue.-The muscular tissue of the heart is intermediate in character between striated and non-striated muscle. Like the non-striated muscle, it is

composed of cells, each containing a single nucleus, and possessing no sarcolemma. But the cells (Fig. 109) are generally short and broad, frequently branched or irregular in shape, and their substance is more or less distinctly striated, like the substance of a striated fibre. A number of such cells are joined by cement substance into sets of anastomosing fibres, which are built up in a complex interwoven manner into the walls of the ventricles and auricles.

p

n

FIG. 109.-CArdiac Fibre CELLS.

Two cells isolated from the heart.", nucleus; 7, line of junction between the two cells;, process joining a similar process of another cell. (Magnified 400 diameters.)

40. Nervous tissue.-The characters of nervous tissue are very different in different parts of the nervous system. We may best begin with the study of a motor nerve— such an one, for example, as that which supplies the biceps muscle.

Like the muscle, the nerve is a compound organ consisting of, (a,) a nerve-case or perineurium (formerly known as the neurilemma1), partitions from which inclose a great number of parallel tubular cavities, each of which contains, (b,) a nerve fibre.

The perineurium, like the perimysium, is composed of connective tissue and supports the scanty vessels of the

1 See note, p. 356.

nerve. It consists of an external layer, which envelops the whole nerve, and, within this, layers disposed concentrically around, and thus forming secondary sheaths for, larger and smaller bundles of nerve fibres. these secondary sheaths smaller and smaller groups are formed until at length partitions, incomplete and of extreme tenuity, are formed between the individual nerve fibres.

Within

41. The nerve fibres, which are the essential elements of the nerve, vary in diameter from 2μ to 12μ. In the living state they are very soft cylindrical rods of a glassy, rather strongly refracting aspect. No limiting membrane is distinguishable from the rest of the substance of the rod, but running through the centre of it a band of somewhat less transparency than the rest may be discerned. At intervals, the length of which varies, but is always many times greater than the thickness of the rod, the nerve fibre presents sharp constrictions, which are termed nodes (Fig. 110. B. n n). Somewhere in the interspace between every two nodes, very careful examination will reveal the existence of a nucleus (Fig. 110, B. nc), invested by more or less protoplasmic substance and lying in the substance of the rod, but close to the surface.

As the fibre dies, and especially if it is treated with certain re-agents, these appearances rapidly change. 1. The outermost layer of the fibre becomes recognisable as a definite membrane, the neurilemma1 (the so-called 'primitive sheath" or sheath of Schwann "). 2. The central band becomes more opaque, and sometimes appears marked with fine longitudinal striæ as if it were composed of extremely fine fibrillæ; it is the neuraxis ("axis cylinder "' or axis fibre" of Remak). 3. Where the neuraxis traverses one of the nodes the neurilemma is seen to embrace it closely, but in the intervals between the nodes a curdy-looking matter, which looks white by reflected light, occupies the space between the neuri

66

This word was formerly used to denote the whole nerve-case, now called perineurium; but its similarity to the word sarcolemma led to great confusion in the minds of students. It is undoubtedly a wholesome rule never to use an old word in a new sense; but the striking similarity between the two words "neurilemma" and "sarcolemma," and between the nerve-fibre sheath and the muscle-fibre sheath, seems an adequate excuse for an exception to the rule.

FIG. 110.-TO ILLUSTRATE THE STRUCTURE OF NERVE FIBRES. A. A nerve fibre seen without the use of reagents, showing the "double contour due to the medulla, and, n, a node. Neither neuraxis nor neuri. lemma can be distinctly seen. (Magnified about 300 diameters.)

B. A thin nerve fibre treated with osmic acid, showing, uc, nucleus with protoplasm, surrounding it, beneath the neurilemma; nn, the two nodes marking out the segment to which the nucleus belongs. (Magnified 400 diameters.)

C. Portion of fibre (thicker than B), treated with osmic acid to show the node n; m, the densely stained medulla; at m' the medulla is seen divided into segments. (Magnified 350 diameters.)

D. Portion of nerve fibre treated to show the passage of the neuraxis, nx, through the node, n; m, the medulla. At nx' the neuraxis is swollen by the reagents employed and large and irregular. (Magnified 300 diameters.) E. Portion of nerve fibre treated with osmic acid, showing the nucleus, nc, embedded in the medulla; c, fine perineurial sheath lying outside the neurilemma, the outline of the latter can only be recognised over the nucleus nc.; the nucleus, nc', belongs to this perineurial sheath. (Magnified 400 diameters.)

F. Portion of nerve fibre deprived of its neurilemma and showing the medulla broken up into separate fragments, m m, surrounding the neuraxis, nx.