LESSON XII.

HISTOLOGY; OR, THE MINUTE STRUCTURE OF THE TISSUES.

I. IN the first chapter (§ 11) attention was directed to the obvious fact that the substance of which the body of a man or other of the higher animals is composed, is not of uniform texture throughout; but that, on the contrary, it is distinguishable into a variety of components which differ very widely from one another, not only in their general appearance, their colour, and their hardness or softness, but also in their chemical composition, and in the properties which they exhibit in the living state.

In dissecting a limb there is no difficulty in distinguishing the bones, the cartilages, the muscles, the nerves and so forth from one another; and it is obvious that the other limbs, the trunk, and the head, are chiefly made up of similar structures. Hence, when the foundations of anatomical science were laid, more than two thousand years ago, these "like" structures which occur in different parts of the organism were termed homoiomera,“ similar parts." In modern times they have been termed "tissues," and the branch of biology which is concerned with the investigation of the nature of these tissues is called Histology.

Histology is a very large and difficult subject, and this whole book might well be taken up with a thorough discussion of even its elements. But physiology is, in ultimate analysis, the investigation of the vital properties

of the histological units of which the body is composed, And even the elements of physiology cannot be thoroughly comprehended without a clear apprehension of the nature and properties of the principal tissues.

2. A good deal may be learned about the tissues without other aid than that of the ordinary methods of anatomy, and it is extremely desirable that the student should acquire this knowledge as a preliminary to further inquiry. But the chief part of modern histology is the product of the application of the microscope to the elucidation of the minute structure of the tissues; and this has had the remarkable result of proving that these tissues themselves are made up of extremely small homoiomera, or similar parts, which are primitively alike in form in all the tissues.

3. Every tissue therefore is a compound structure: a multiple of histological units, or an aggregation of histological elements; and the properties of the tissue are the sum of the properties of its components.

The distinctive character of every fully formed tissue depends on the structure, mode of union, and vital properties of its histological elements when they are fully formed. But each tissue can be traced back to a young or embryonic condition, in which it has no characteristic properties, and in which its histological elements are so similar in structure, mode of union, and vital properties to those of every other embryonic tissue, that our present means of investigation do not enable us to discover any difference among them.

4. These embryonic, undifferentiated, histological elements, of which every tissue is primitively composed, or, as it would be more correct to say, which, in the embryonic condition, occupy the place of the tissues, are technically named nucleated cells. The colourless blood corpuscle (Lesson III. § 6) is a typical representative of such a cell. And it is substantially correct to say (1) that the histological elements of every tissue are modifications or products of such cells; (2) that every tissue was once a mass of such cells more or less closely packed together; and (3) that the whole embryonic body was at one time nothing but an aggregation of such cells.

5. The body of a man or of any of the higher animals

in fact commences as an ovum or egg. This (Fig. 87) is a minute transparent spheroidal sac, of an inch in diameter in man, which contains a similarly spheroidal mass of protoplasm, in which a single large nucleus is imbedded.

The first step towards the production of all the complex organization of a mammal out of this simple body is the division of the nucleus into two new nuclei which recede from one another, while at the same time the protoplasmic body becomes separated, by a narrow cleft which runs between the two nuclei, into two masses, or blastomeres, (Fig. 88) one for each nucleus. By the repetition of the process the two blastomeres give rise to four, the four to eight, the eight to sixteen, and so on, until the embryo is

a, Granular protoplasm; b, nucleus, called "germinal vesicle;" c, nucleolus, called "germinal spot."

an aggregate of numerous small blastomeres, or nucleated cells. These grow at the expense of the nutriment supplied from without, and continue to multiply by division according to the tendencies inherent in each until, long before any definite tissue has made its appearance, they build themselves up into a kind of sketch model of the developing animal, in which model many of, if not all the future organs are represented by mere aggregates of undifferentiated cells.

6. Gradually, these undifferentiated cells become changed into groups or sets of differentiated cells, the cells in one set being like each other, but unlike those of other sets. Each set of differentiated cells constitutes a "tissue," and each tissue is variously distributed among the several

organs, each organ generally consisting of more than one tissue.

And this differentiation in structure is accompanied by a change of properties. The undifferentiated cells are, as far as we can see, alike in function and properties as they are alike in structure. But coincident with their differentiation into tissues, a division of labour takes place, so that in one tissue the cells manifest special properties

FIG. 88. THE SUCCESSIVE DIVISION OF THE MAMMALIAN OVUM INTO BLASTOMERES. Somewhat diagrammatic.

a, division into two; b, into four; c, into eight, and d, into several blastoineres. The clear ring seen in each case is the zona pellucida, or membrane investing the ovum.

and carry on a special work; in another they have other properties, and other work; and so on.

The principal tissues into which the undifferentiated cells of the embryo become differentiated, and which are variously built up into the organs and parts of the adult body, may be arranged as follows.

(1.) The most important tissues are the muscular and

nervous tissues, for it is by these that the active life of the individual is carried on.

(2.) Next come the epithelial tissues, which, on the one hand, afford a covering for the surface of the body as well as a lining for the various internal cavities of the body: and, on the other hand, carry on a great deal of the chemical work of the body, inasmuch as they form the essential part of the various glandular organs of the body.

(3.) The remaining principal tissues of the body, namely the so-called connective tissue, cartilaginous tissue and osseous or bony tissue, form a group by themselves, being all three similar in their fundamental structure, and all three being, for the most part, of use to the body for their passive rather than for their active qualities. They chiefly serve to support and connect the other tissues. These principal or fundamental tissues are often arranged together to form more complex parts of the body, which are sometimes spoken of, though in a different sense, as tissues. Thus various forms of connective tissue are built up with some muscular tissue and nervous tissue, to form the blood-vessels of the body (see Lesson II.), which are sometimes spoken of as vascular tissue." So again, a certain kind of epithelial tissue, known as epidermis," together with connective tissue, blood-vessels and nerves, forms the skin or tegumentary tissue; a similar combination of epithelium with other tissues constitutes the mucous membrane lining the alimentary canal, and also occurs in the so-called "glandular" tissue.

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We may confine our attention here to the principal tissues properly so-called.

7. Epithelial tissue. A good example of this tissue is to be found in the epidermis of the skin, which, as we have seen (Lesson V.), consists of the superficial epidermis which is non-vascular and epithelial in nature, and of the deep derma, which is vascular, and is indeed chiefly composed of connective tissue carrying blood-vessels and nerves. And in all the mucous membranes there is a similar superficial epithelial layer, which is here simply called epithelium, and a deep layer, which similarly consists of connective tissue carrying blood-vessels and nerves and may also be spoken of as derma.

8. If a piece of fresh skin is macerated for some time in