dark, an apparent reversal being thereby produced in the striæ. This reversal is due to the enlargement of the rows of dark dots, and the formation by their juxtaposition and blending of dark disks, whilst the muscular substance between these disks has by contrast a bright appearance.

The wing-muscles of insects are easily broken up into very fine fibres or fibrils, which also show alternate dark and light striæ. The number and relative thickness of these differ, however, considerably, according to the amount of stretching of the fibres (fig. 75). Musclerods are not seen in these fibres.

FIG. 75.-FIBRES OF THE WING-MUSCLES OF AN INSECT.

The fibres are in different conditions of extension, from A least extended, to D most extended. e, e, chief substance of the fibre; m, m, intermediate lines or disks; the light bands, bc, on either side of these only come to view when the fibre is sufficiently stretched (C); with further extension (D), the middle of the dark band appears lighter, h.

In muscular tissue which has been hardened in alcohol and certain other reagents, the structural appearances are a good deal altered from those of the living muscle, although the cross-striæ are still very obvious. There is also a considerable tendency for the fibres to split up longitudinally into fibrils, and by some authorities the finest of such fibrils are regarded as the ultimate elements of the fibre. Certain other reagents, such as dilute hydrochloric acid, cause a transverse splitting of the fibres into disks, and these effects of reagents led Bowman to form the opinion that the muscular substance may be in reality composed of minute prismatic particles set side by side in rows or planes to form the disks, and adhering end to end longitudinally to form the fibrils. To these constituent particles of the muscular substance he gave the name of sarcous elements.'

6

When living muscular fibres are examined by polarised light, the whole of the muscular substance except the muscle-rods is seen to be doubly refracting, looking bright in the dark field produced by crossing the axes of the Nichol's prisms. Contracted muscle and dead muscle show, however, alternate bands of dark and light under those circumstances.

Ending of muscle in tendon.-A small tendon-bundle passes to

F

each muscular fibre and becomes firmly united with the sarcolemma, which extends over the end of the fibre (fig. 76). Further, the areolar tissue between the tendon-bundles is continuous with that which lies between the muscular fibres, so that the connection of a muscle to its tendon is very firm.

Blood-vessels of muscle.-The capillaries of the muscular tissue are very numerous. They run, for the most part, longitudinally, with transverse branches, so as to form long oblong meshes (fig. 77). In the red muscles of the rabbit and hare, the transverse capillaries have small dilatations upon them. No blood-vessels ever penetrate the

sarcolemma.

Lymphatic vessels, although present in the connective-tissue sheath (perimysium) of a muscle, do not penetrate between its component fibres.

The nerves of voluntary muscles pierce the sarcolemma and terminate in a ramified expansion known as an end-plate (see Lesson XIX.).

Voluntary muscular fibres are developed from embryonic cells of the mesoderm, which become elongated, and the nuclei of which become multiplied, so as to produce long multi-nucleated fusiform or cylindrical fibres. These become cross-striated at first along one side, the change gradually extending around the fibre and also towards the centre; but the middle of the fibre, to which the nuclei are at first confined, remains for some time unaltered (fig. 78). Eventually the change in structure extends to this also, and the nuclei pass gradually to occupy their ordinary position under the sarcolemma, which by this time has become formed.

Involuntary or plain muscular tissue is composed of long, somewhat flattened, fusiform cells (fig. 79), which vary much in length, but are usually not more than inch long. Each cell has an oval or

rod-shaped nucleus, which shows the usual intra-nuclear network and commonly one or two nucleoli. The cell-substance is longitudinally striated, but does not exhibit cross-striæ like those of voluntary muscle. There appears to be a delicate sheath to each cell. There is a little intercellular cementing substance uniting the cells together, and which can be stained by nitrate of silver. The fibres are collected into fasciculi.

Plain muscular tissue is found chiefly in the walls of hollow viscera ; thus it forms the muscular coat of the whole of the alimentary canal below the œsophagus, and occurs abundantly in the muscular coat of that tube also, although it is here intermixed with cross-striated muscle; it is found also in the mucous membrane of the alimentary canal; in the trachea and its ramifications; in the urinary bladder and ureters; in the uterus, Fallopian tubes, and ovary; in the prostate, the spleen, and muscle of Müller in the orbit, and in the ciliary muscle, and iris. The walls of gland-ducts also contain it, and the middle coat of the arteries, veins, and lymphatics is largely composed of this tissue. It occurs also in the skin, both in the secreting part of the sweat-glands, and in small bundles attached to the hair-follicles; in the scrotum it is found abundantly in the subcutaneous tissue (dartos).

The muscular tissue of the heart constitutes a special variety of involuntary muscular tissue (cardiac), and will be described along with that organ.

LESSON XVII.

STRUCTURE OF NERVE-FIBRES.

1. TEASE a piece of fresh nerve in saline solution, injuring the fibres as little and obtaining them as long and straight as possible. Study the medullated fibres, carefully noticing all the structures that are visible-viz., nodes of Ranvier, nuclei of primitive sheath, double contour of medullary sheath, medullary segments, &c. Measure the diameter of half a dozen fibres. Draw a short length of a fibre very exactly.

2. Prepare a piece of the sympathetic nerve in the same way. Measure and sketch as before.

3. Separate (in dilute glycerine or Farrant) into its fibres a small piece of nerve that has been twenty-four hours in per cent. osmic acid. The nerve should have been moderately stretched on a piece of cork by means of pins before being placed in the acid. Keep the fibres as straight as possible and only touch them near their ends with the needles. Sketch two portions of a fibre under a high power, one showing a node of Ranvier and the other a nucleus of the primitive sheath. Look for fibres of Remak. Measure the length of the nerve-segments between the nodes of Ranvier.

4. Mount in Canada balsam sections of a nerve which has been hardened in picric acid and stained with picro-carmine. The nerve should have been stretched out before being placed in the hardening solution. Examine the sections first with a low and afterwards with a high power. Notice the lamellar structure of the perineurium, the varying size of the nerve-fibres, the axis cylinder in the centre of each fibre, &c. Measure the diameter of five or six fibres, and sketch a small portion of one of the sections.

Nerve-fibres are of two kinds, medullated and non-medullated. The cerebro-spinal nerves and the white matter of the nerve-centres are composed of medullated fibres; the sympathetic and its branches is chiefly made up of non-medullated.

The medullated or white fibres are characterised, as their name implies, by the presence of the so-called medullary sheath or white substance. This is a layer of soft substance, chiefly of a fatty nature, which encircles the essential part of a nerve-fibre, viz. the axis-cylinder. Outside the medullary sheath is a delicate but tough homogeneous membrane, the primitive sheath or nucleated sheath of Schwann, but