Endothelial membranes (fig. 13) are quite similar in structure to mesothelial. They are usually elongated cells of irregular outline and serrated borders.
| Fig. 13.—Endothelial Cells from the Interior of an Artery. |
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.
(T. G. Br.)
EPITOME (Gr. ἐπιτομή, from ἐπιτέμνειν, 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. ἐποχή, holding in suspense, a pause, from ἐπέχειν, 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.