ANIMAL.] HISTOLOGY 9 end as to cause the muscular substance to present both a transversely and a longitudinally striated appearance. Sometimes (as in the fibre shown in the figure) the substance of each muscle rod is partly collected into a swelling or knob at either end, and these knobs so act singly and collectively upon the light which passes through the muscular substance as to cause a brighter appearance in their neighbourhood. In this way bright bands seem to cross the muscular sub stance alternating with the dimmer intermediate portions, and the appearance of transverse striation is much intensi fied. This is still more the case when the muscle contracts, for the contraction is accompanied by an accumulation of the substance of the rods towards their ends, and an apparent blending of these into a dark transverse band or rather a series of dark transverse bands, which, reflect ing from their surfaces the light which is passing through the muscle, cause the whole of the substance between them to appear much brighter than they are themselves. There are other muscular fibres in the insect which present an entirely different appear ance. In these (fig. 20) the fibres, which FlG 20 _ Fibre of are very fine, are wholly made up of alter- wing-muscle of nating bands of dark and light substance. They are far less like the voluntary muscular fibres of mammals than are the others, and there are no rod-like structures to be seen in them. We find muscular tissue, like the other tissues, appearing already in the lowest of the Metazoa. In sponges the orifices of the water canals are in many cases capable of being closed partially or wholly when the organism is irri tated. The researches of F. E. Schulze have shown that these orifices are encircled by long fusiform cells which appear to be modifications of some of the ordinary ramified cells of the jelly-like tissue. The substance of these cells seems to be undifferentiated, and it cannot be conclusively affirmed that they are of muscular nature, but at least they seem to subserve the function of muscular tissue. But in the very next division the Hydromedusce the muscular cells are already so much differentiated as to exhibit both longitudinal and transverse striation. Thus many of the cells which form the muscular layer of the sub-umbrella of the Medusa? are long fusiform cells (fig 21, A) with an elongated nucleus in the centre, and gradually tapering ends, and their substance is striated, as just remarked, both transversely and longitudinally. Sometimes there is a con siderable amount of unaltered protoplasm in the middle of the fibre around the nucleus (fig 21, B), and this nucleated protoplasm may then project between the epithelial cells FIG. 21. Muscular cells of Jelly-fish. (Ilcrtwig.) of the ectoderm. In every case the muscular fibres are in close contiguity with the attached ends of the ectodermal epithelium, and are with justice reckoned as a part of the ectoderm. In the higher coclcnterates the muscular tissue tends to lose its connexion with the ectoderm and to become embedded in the jelly-like mesoderm, but the connexion is not wholly lost in any. In Hydra, on the other hand, the muscular tissue is represented only by simple longitudinal fibres, which are either direct prolongations of the tapering ends of some of the ectoderm cells (Kleinenberg) or are embedded in the enlamed attached end of the cells 22. Muscu- fibres of mol- (Kolliker, Korotneff). In other invertebrates the muscular tissue is nearly always in the form of long cylindrical or flattened, tapering or uniform, uninucleated, longitudinally- striated fibres, which may possess a membrane, and a central strand of undifferentiated protoplasm (fig. 22, B). In some cases a transverse striation may be detected (fig. 22, A), but more commonly the muscular fibres, especially in echinoderms, worms, and molluscs, ex- A hibit a peculiar double oblique striation (fig. 23), so that an appearance of intercrossing lines is thereby produced. The obliquely striated fibres seem to take the place, in many of these comparatively sluggish ani mals, of the more active, transversely striated tissues. With the exception of the appear ance mentioned, they resemble the plain muscular fibres in structure, but they are capable of more energetic contraction than the latter. The Nervous Tissues of Animals. The nervous tissue of vertebrates is composed firstly of cells the nerve-cells or ganglion- cells, and secondly of nerve-fibres. Most of the nerve-fibres possess a sheath formed of nucleated cells wrapped around the fibre,. and in this sheath a peculiar white fatty luscs - so-called medullary substance is accumulated in some fibres, so that they are distinguished from the others as the white or medullated fibres. There is reason to believe that every nerve-fibre is connected with at least one nerve-cell, and conversely, that every nerve- cell is connected directly or indirectly with one or more nerve-fibres. Nerve-cells are generally comparatively large solid-looking corpuscles, with a relatively large nucleus and nucleolus, and every developed nerve- cell has either one or two or a greater number of processes, which may or may not be ramified. It is certain that from many nerve-cells one process of the cell passes into and becomes a nerve-fibre. 1 Nerve-cells are always traversed by ex quisitely fine fibrils, nerve-fibrils, and these pass out from the cell into its processes. Apart from any sheath which it may possess, a nerve-fibre is composed of one or more nerve-fibrils, which are em bedded in a soft interfibrillar sub stance. The nervous tissue of ver tebrates is developed from that part of the ectoderm which occupies the middle of the dorsal surface of the embryo. In the bird and mam mal the epithelial cells in this situation become cut off from the general ectoderm by the forma tion of a groove which subse quently closes over and forms a canal the neural canal. The innermost ectoderm cells (fig. 24, B) which form the wall of this canal acquire cilia at the end which is turned towards the cavity, while the other end of each cell is pro longed into branching processes which collectively form a network amongst the deeper lying cells of the wall. The latter multiply considerably, and moreover groups of them grow out from the sides of the neural canal as the roots of the nerves. The nerve-fibres themselves seem to be formed either by the outgrowth of undivided processes from these cells of the neural canal, or by the junction of one elongated 1 The term nerve-fibre is here employed to denote the essential part of the nerve, corresponding to the "axis-cylinder" of vertebrate histology. XII. 2 FIG. 23. Part of an obliquely stri ated muscular fibre. (From Schwalbc.) A B FIG. 24. Nerve - epithelium cells A, of Medusa; R, from central canal of spinal cord of
vertebrate.
