Page:EB1911 - Volume 09.djvu/739

 terminal portions of the ramifications and extend upwards to varying degrees. Thus in a typical acinous gland the cells are restricted to the final alveoli. The remaining tubes are to be considered mainly as ducts. In tubulo-alveolar glands the secreting epithelium lines the alveus as well as the terminal tubule.

The gland cells are all placed upon a basement membrane. In many instances this membrane is formed of very thin flattened cells, in other instances it is apparently a homogeneous membrane, and according to some observers is simply a modified part of the basal surface of the cell, while according to others it is a definite structure distinct from the epithelium.

In the secretory portion of the gland and in the smaller ducts the epithelial layer is one cell thick only. In the larger ducts there are two layers of cells, but even here the surface cell usually extends by a thinned-out stalk down to the basement membrane.

The detailed characters of the epithelium of the different glands of the body are given in separate articles (see, &c.). It will be sufficient here to give the more general characters possessed by these cells. They are cubical or conical cells with distinct oval nuclei and granular protoplasm. Within the protoplasm is accumulated a large number of spherical granules arranged in diverse manners in different cells. The granules vary much in size in different glands, and in chemical composition, but in all cases represent a store of material ready to be discharged from the cell as its secretion. Hence the general appearance of the cell is found to vary according to the previous degree of activity of the cell. If it has been at rest for some time the cell contains very many granules which swell it out and increase its size. The nucleus is then largely hidden by the granules. In the opposite condition, i.e. when the cell has been actively secreting, the protoplasm is much clearer, the nucleus obvious and the cell shrunken in size, all these changes being due to the extrusion of the granules.

Endothelium and Mesothelium.—Lining the blood vessels, lymph vessels and lymph spaces are found flattened cells apposed to one another by their edges to form an extremely thin membrane. These cells are developed from the

middle embryonic layer and are termed endothelium. A very similar type of cells is also found, formed into a very thin continuous sheet, lining the body-cavity, i.e. pleural pericardial, and peritoneal cavities. These cells develop from that portion of the mesoderm known as the mesothelium, and are therefore frequently termed mesothelial, though by many they are also included as endothelial cells.

A mesothelial cell is very flattened, thus resembling a squamous epithelial cell. It possesses a protoplasm with faint granules and an oval or round nucleus (fig. 12). The outline of the cell is irregularly polyhedral, and the borders may be finely serrated. The cells are united to one another by an intercellular cement substance which, however, is very scanty in amount, but can be made apparent by staining with silver nitrate when the appearance reproduced in the figure is seen. By being thus united together, the cells form a continuous layer. This layer is pierced by a number of small openings, known as stomata, which bring the cavity into direct communication with lymph spaces or vessels lying beneath the membrane. The stomata are surrounded by a special layer of cubical and granular cells. Through these stomata fluids and other materials present in the body-cavity can be removed into the lymph spaces.

Endothelial membranes (fig. 13) are quite similar in structure to mesothelial. They are usually elongated cells of irregular outline and serrated borders.

By means of endothelial or mesothelial membranes the surfaces of the parts covered by them are rendered very smooth, so that movement over the surface is greatly facilitated. Thus the abdominal organs can glide easily over one another within the peritoneal cavity; the blood or lymph experiences the least amount of friction; or again the friction is reduced to a minimum between a tendon and its sheath or in the joint cavities. The cells forming these membranes also possess further physiological properties. Thus it is most probable that they play an active part in the blood capillaries in transmitting substances from the blood into the tissue spaces, or conversely in preventing the passage of materials from blood to tissue space or from tissue space to blood. Hence the fluid of the blood and that of the tissue space need not be of the same chemical composition.

EPITOME (Gr. , from  , to cut short), an abridgment, abstract or summary giving the salient points of a book, law case, &c., a short and concise account of any particular subject or event. By transference epitome is also used to express the representation of a larger thing, concrete or abstract, reproduced in miniature. Thus St Mark’s was called by Ruskin the “epitome of Venice,” as it embraces examples of all the periods of architecture from the 10th to the 19th centuries.

EPOCH (Gr. , holding in suspense, a pause, from  , to hold up, to stop), a term for a stated period of time, and so used of a date accepted as the starting-point of an era or of a new period in chronology, such as the birth of Christ. It is hence transferred to a period which marks a great change, whether in the history of a country or a science, such as a great discovery or invention. Thus an event may be spoken of as “epoch-making.” The word is also used, synonymously with “period,” for any space of time marked by a distinctive condition or by a particular series of events.

In astronomy the word is used for a moment from which time is measured, or at which a definite position of a body or a definite relation of two bodies occurs. For example, the position of a body moving in an orbit cannot be determined unless its position at some given time is known. The given time is then the epoch; but the term is often applied to the mean longitude of the body at the given time.

EPODE, in verse, the third part in an ode, which followed the strophe and the antistrophe, and completed the movement; it was called  by the Greeks. At a certain moment the choirs, which had chanted to right of the altar or stage and then to left of it, combined and sang in unison, or permitted the coryphaeus to sing for them all, standing in the centre. When, with the appearance of Stesichorus and the evolution of choral lyric, a learned and artificial kind of poetry began to be cultivated in Greece, a new form, the  , or epode-song, came into existence. It consisted of a verse of trimeter iambic, followed by a dimeter iambic, and it is reported that, although the epode was carried to its highest perfection by Stesichorus, an earlier poet, Archilochus, was really the inventor of this form. The epode soon took a firm place in choral poetry, which it lost when that branch of literature declined. But it extended beyond the ode, and in the early dramatists we find numerous examples of monologues and dialogues framed on the epodical system. In Latin poetry the epode was cultivated, in conscious archaism, both as a part of the ode and as an independent branch of poetry. Of the former class, the epithalamia of Catullus, founded on an imitation of Pindar, present us with examples of strophe, antistrophe and epode; and it has been observed that the celebrated ode of Horace, beginning Quem virum aut heroa lyra vel acri, possesses this triple character. But the word is now mainly familiar from an experiment of Horace in the second class, for he entitled his fifth book of odes Epodon liber or the Book of Epodes. He says in the course of these poems, that in composing them he was introducing a new form, at least in Latin literature, and that he was imitating the effect of the iambic distichs invented by Archilochus. Accordingly we find the first ten of these epodes