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is a story, which is credible enough, though it may not be true, of a practical joker, who, seeing a discharged veteran carrying home his dinner, suddenly called out Attention!" whereupon the man instantly brought his hands down, and lost his mutton and potatoes in the gutter. The drill had been thorough, and its effects had become embodied in the man's nervous structure.
The possibility of all education (of which military drill is only one particular form) is based upon the existence of this power which the nervous system possesses, of organizing conscious actions into more or less unconscious, or reflex, operations. It may be laid down as a rule, that if any two mental states be called up together, or in succession, with due frequency and vividness, the subsequent production of the one of them will suffice to call up the other, and that whether we desire it or not.
The object of intellectual education is to create such indissoluble associations of our ideas of things, in the order and relation in which they occur in nature; that of a moral education is to unite as fixedly, the ideas of evil deeds with those of pain and degradation, and of good actions with those of pleasure and nobleness.
26. The sympathetic system consists chiefly of a double chain of ganglia, lying at the sides and in front of the spinal column, and connected with one another, and with the spinal nerves, by commissural cords. From these ganglia, nerves are given off which for the most part follow the distribution of the vessels, but which, in the thorax and abdomen, form great networks, or plexuses, upon the heart and about the stomach. It is probable that a great proportion of the fibres of the sympathetic system is derived from the spinal cord; but others also, in all probability, originate in the ganglia of the sympathetic itself. The sympathetic nerves influence the muscles of the vessels generally, and those of the heart, of the intestines, and of some other viscera : and it is probable that their ganglia are centres of reflex action to afferent nerves from these organs. But many of the motor nerves of the vessels are, as we have seen, under the influence of particular parts of the spinal cord, though they pass through sympathetic ganglia.
HISTOLOGY; OR, THE MINUTE STRUCTURE OF THE TISSUES.
I. THE various organs and parts of the body, the working of which has now been described, are not merely separable by the eye and the knife of the anatomist into membranes, nerves, muscles, bones, cartilages, and so forth; but each of them is, by the help of the microscope, susceptible of a finer analysis, into certain minute constituents which, for the present, may be considered the ultimate structural elements of the body.
2. There is a time when the human body, or rather its rudiment, is of one structure throughout, consisting of a more or less transparent matrix, very similar in nature to the substance of which the white blood-corpuscles are composed, and often called protoplasm, through which are scattered minute rounded particles of a different optical aspect. These particles are called nuclei; and as the matrix, or matter in which these nuclei are imbedded, readily breaks up into spheroidal masses, one for each nucleus, and these investing masses easily take on the form of vesicles or cells, this primitive structure is called cellular, and each cell is said to be nucleated.
The material of the body when in this stage of growth is often spoken of as indifferent tissue. A very fair idea of its nature may be formed by supposing a multitude of white blood-corpuscles to be collected together into a soft but yet semi-solid mass.
In the present use of the term any distinct mass of protoplasm or living material may be called a cell. In the vast majority of cases, however, the cell contains a nucleus,
distinguished as has just been said from the cell-substance in which it lies. Very frequently, but by no means always, the outer layer of the cell-substance is hardened into a distinct casing or envelope, the cell-wall, the cell then becoming an undeniable vesicle, and the cell-substance being often spoken of as the cell contents. The cellsubstance may remain as soft semi-solid protoplasm, or may be hardened in various ways, or may be wholly or partially liquefied; in the latter case a cell-wall is naturally always present.
A, vertical section of a layer of epidermis, or epithelium, from its free to its deep surface. B, lateral views of the cells of which this layer is composed at different heights: a, cell in the deepest layer, and therefore most recently formed and least altered; b, cell higher up, and therefore somewhat changed; c, d, cells still more changed, and much flattened. C, scales such as d viewed from their flat sides. (Magnified about 250 diameters.)
As development goes on, the nuclei simply increase in number by division and subdivision, without undergoing any marked change;1 but the substance in which they are
1 Each nucleus divides into two, and each half soon grows up into the size of the parent nucleus. While this is going on the matrix round the nuclei also divides, each new nucleus having a quantity of matrix allotted to it, so as to form a new cell exactly like the old one, from which it sprang.
imbedded, becomes very variously modified, both chemically and structurally, and gives rise to those peculiarities by which completely formed tissues are distinguished from one another.
3. In the adult body the simplest forms of tissue, i.e. those in which the matrix has been least changed, are perhaps the various kinds of epithelium (including the epidermis).
These are distinctly cellular in nature, that is, the portion of the matrix belonging to each nucleus can, with a little pains, be recognized as distinct from the portions belonging to the other nuclei. In fact they differ from white blood corpuscles chiefly in two points: firstly, the matrix of each cell becomes more or less chemically changed so as to lose its soft protoplasmic nature (and at the same time its power of executing amoeboid movements); and, secondly, takes on a rigid definite form, which may or may not be globular. These epithelial tissues are constantly growing in their deepest parts, and are, as constantly, being shed at their surfaces.
The deep part consists of a layer of such globular, nucleated cells as have been mentioned, the number of which is constantly increasing by the spontaneous division of the nuclei and cells. The increase in number thus effected causes a thrusting of the excess of cell population towards the surface; on their way to which they become flattened, and their walls acquire a horny texture. Arrived at the surface, they are mere dead horny scales, and are thrown off (Fig. 86).
Epithelium of the kind just described is called squamous. It is found in the mouth, and its scales may always be obtained in abundance by scraping the inside of the lip.
Epidermis consists of exactly similar cells, except that the conversion of the topmost cells into horny scales is still more complete. The nucleus, too, is eventually lost. The deep layers of epidermis, consisting of softer cells not yet flattened or made horny, often form quite distinct part, and these are often spoken of as the rete mucosum. (See Fig. 32, b; Fig. 88, C, d.)
In other parts of the alimentary tract, as in the intestines, the full-grown epithelial cells are placed side by side with one another, and perpendicular to the surface of
the membrane. Such epithelium is called cylindrical (Fig. 46, b, b'), or columnar.
In some places, such as in the gastric glands, in some parts of the kidney, in the ureters and elsewhere, the epithelial cells remain globular or spheroidal.
Squamous epithelium generally consists of many layers of cells, one over the other; in other forms of epithelium there are few, in some cases apparently only two layers.
FIG. 87.-CILIATED EPITHELIUM.
7, the submucous vascular tissue; b, the deep layer of young epithelium cells; c, the cylindrical full-grown cells, with (d) the cilia. (Magnified about 350 diameters.)
Ciliated epithelium is usually of the cylindrical kind, and differs from other epithelium only in the circumstance that one or more incessantly vibrating filaments are developed from the free surface of each cell. (See Lesson VII. § 3.)
4. In certain regions of the integument, the epidermis becomes metamorphosed into nails and hairs.
Underneath each nail the deep or dermic layer of the integument is peculiarly modified to form the bed of the nail. It is very vascular, and raised up into numerous parallel ridges, like elongated papilla (Fig. 88, B, C). The surfaces of all these are covered with growing epidermic cells, which, as they flatten and become converted into horn, coalesce into a solid continuous plate, the nail. At the hinder part of the bed of the nail, the integument forms a deep fold, from the bottom of which, in like manner,