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

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

, granular protoplasm around the nucleus.

lon-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. n, nucleus; 7, line of junction between the two cells; P, 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 nervesuch 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. Within 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.

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 fibrille; 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

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, c, 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; 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 x' 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, n c', 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, x.

lemma and the neuraxis. This is the medulla (the socalled "white substance of Schwann”) largely composed of a complex fatty substance, often spoken of as myelin. If the neurilemma of a fresh fibre is torn, the myelin flows out and forms irregular lumps as if it were viscous. The medulla is broken, by oblique lines (Fig. 110, C. m), extending from the neuraxis to the neurilemma, into segments, the faces of which are obliquely truncated and fit closely against one another. These may be seen even in quite fresh and living nerve fibres. 4. The internodal nucleus is more sharply defined; and it will be seen to be attached to the inner surface of the neurilemma.

The motor nerve, proceeding to its muscle, enters the perimysium (with which the superficial layer of the perineurium becomes continuous), and divides in the perimysial septa into smaller and smaller branches, each of which contains the continuation of a certain number of the fibres of the nerve trunk, bound up into a bundle by themselves. In these larger ramifications of the nerve trunk there is no branching of the nerve fibres themselves (at any rate as a rule), but merely a separation of the compound nerve cord. In the finest branches, however, the nerve fibres themselves may divide; the division, which always takes place at a node, is generally dichotomous-that is, one fibre divides into two, each of these again into two, and so on. These finest branches consisting of one or two nerve fibres, or of one only, with a very delicate perineurial envelope (Fig. 110, E. c), pass to some single muscle fibre, and each nerve fibre applies itself to the outer surface of the sarcolemma. At this point, if it has not done so before, the medulla disappears, the neurilemma becomes continuous with the sarcolemma, and the neuraxis passes into a disc of protoplasmic substance containing many nuclei, which is interposed between the striated muscle substance and the sarcolemma at this point, thus forming what is called a motor plate or end-plate.1 Before ending the neuraxis divides and its divisions anastomose freely, but the exact relations of the various parts of the endplate to the muscle-substance have not yet been clearly made out. The whole appears to constitute an apparatus This is the arrangement in most vertebrated animals. In the frog the neuraxis branches out without entering a distinct motor or end-plate.