becoming again colorless in the sacro-vertebral angle, which is a little more raised, and resuming its cruoric appearance in the iliac veins, especially in the inner ones; the clots in the pulmonary veins are always very dark, on account of their inclined position. By turning the corpse over, while these clots are forming, their position is changed, and mixed clots of an opposite composition obtained.

It is plain how useful and important these facts may be, in legal medecine, for instance, by deciding the position in which a corpse has lain during 24 hours after death. They are all the result of that singular property by which the internal coat of the vessels prevents coagulation.

This is not the only property of the vascular walls; it is observed that coagulation in the vessels produces a clot, but little or no serum is found: this is owing to the fact that when the arterial coats lose their properties as living tissues, the fluid part of the blood predominates; either because these coats, being no longer living, cannot effect those natural changes of absorption, etc.; or because the separation of the fibrine has left the other albuminous elements of the blood in a state of composition favorable to their exudation, as occurs in the living body, and, by a similar mechanism, in certain forms of œdema and albuminuria.

PART FIFTH.

EPITHELIAL GLOBULES AND EPITHELIAL SURFACES IN GENERAL.

WE have studied the nerve globule, which by its prolongations places the globular elements of the organism, or of their derivatives, in relation with each other (reflexes); and the muscle, which, obeying the motor prolongations of the nerve globule, serves to modify mechanically the relations between the different parts of the organism to each other, or to the outer world; we have seen that, for this purpose, there are numerous mechanical apparatus attached to the muscle (bones, tendons, ligaments, etc.); we have, finally, studied the blood globule, and the blood, which, loaded with the new materials absorbed by certain surfaces of the organism, carries these former into the deeper tissues, while it draws to the excretory surfaces the products of decomposition and of the interior combustion of the organism. We have now, therefore, to study the physiology of these surfaces, that is, the epithelial globules.

Anatomically speaking, the epithelial globule is already known to us; what especially distinguishes it is its relation to the free surfaces of the body; its surfaces are, in fact, formed of membranes, composed of a more or less close padding of connective and elastic fibres, and are covered by an element of which modern anatomy alone has conceived the importance, epithelium.

It was long believed that the first organ which appears in the embryo, is the nervous system. Modern histological research has proved that the first layer of blastoderm is of an epithelial nature: this layer, in its subsequent development, becomes the intestinal epithelium, the first organic membrane which distinguishes the individual. The importance of the

epithelium, particularly that of the digestive organs, is thus shown by its early formation; its dimensions, in the embryo, are immense. We find that, by the thickness of its layers, it blocks up the opening of the small intestine in the fœtus, and even in the adult it is sometimes 4 or 5 times thicker than the membrane which supports it.

I. GENERAL ANATOMY OF THE EPITHELIUMS.

ANATOMISTS recognize two distinct forms of epithelium, pavement and columnar epithelium; it is only in their extremes, however, that they differ so much, there being intermediate forms between them. The principal epithelium, for instance, that which forms the essential parenchyma of the glands, is neither the pavement nor the columnar epithelium; it is a kind of spherical globule.

The membranes, whose free surface is coated with epithe lium, belong to two categories: 1, serous membrane, generally forming closed cavities; 2, integumentary membrane (either internal or external). The distinguishing characteristics observed in these membranes are dependent on the nature of their epithelium.

A. Serous Membrane.

A

B

The class of epithelium spread on the surface of the serous membranes, is the pavement form (Fig. 56, A). It is generally a single layer of cells which, in consequence of reciprocal deformation (being crowded together), have flattened into angular, polygonal disks: such is the epithelium of the abdominal serum; the case is the same with that of the pericardium, of the arachnoid membranes, and of all the serous membranes called visceral. The epithelium which lines the inner surface of the blood-vessels, and the cavities of the heart (endocardium) is also of this kind. The epithelium covering the articulating cavities is also pavement, but composed of several layers;

Fig. 56.

C

Various forms of epitheliums.*

* A, Pavement epithelium. B, Columnar epithelium. C, Stratified epithe

there are, beside, gaps in this epithelial casing (synovial), where the cartilages come into contact, and where there is, consequently, the strongest pressure. The opinion can no longer be held that the fibrous substratum of the serous membrane alone ceases to occur at the level of the articulating cartilages, while a layer of epithelium remains on these articulating (cartilaginous) surfaces. The articulating surfaces are closed cavities, but their whole inner surface is not lined with epithelium.

B. Integumentary Membranes.

Many organisms possess only one external integument; this is the case with vegetables. But animals, under their cutaneous surfaces, have internal surfaces, communicating with the exterior; these are mucous membranes.

a. External Integuments. - The epithelium of these surfaces is composed of numerous layers: on the surface are found flattened cells, while globular forms prevail in the deeper layers; these latter elements exhibit those signs of life which characterize the epitheliums; in fact, what is commonly called epidermis, the most superficial layer of the skin, is not living epithelium, but a dead body, a horny substance as impermeable as India-rubber. But below, is found a soft succulent membrane, which has all the features of the epitheliums of the mucous membranes, and was formerly called Malpighi's net (rete malpighianum); this, properly speaking, constitutes the living epidermis: it forms a continuous covering to the surface of the dermis.

b. Internal or mucous Integuments. - All the sub-diaphragmatic part of the intestinal canal, the beginning of the trachea and of the genital organs, and their course as far as

* a, Body of the cells. c, Cilia. b, Molecules floating in the ambient fluid, and driven by the cilia in the direction of the upper arrow, in which direction they are erect, while in that of the lower arrow they appear bent. (Valentin.)

the internal genital organs, properly so called, exhibit the features of the external integuments, if the essential element of the mucous membrane, epithelium, be taken into account; the pavement form being always found on the surface, and the globular forms beneath. But if we penetrate these organs more deeply, we find that the epithelium changes its form, and becomes cylindrical. Thus, in the epithelium which covers the uterus, the spermatic organs, the stomach, the intestine, and the trachea below the vocal cords, we recognize certain general features, such as the cylindrical or conical form of cells, and the constant presence of the nuclei (Fig. 58); and also, characteristic peculiarities, of which the most important is the existence in some of them

I

K

of ciliated prolongations, with Fig 58. Columnar or cylindrical which their free surfaces are provided, having a continual vibra

cells of the intestinal mucous membrane. (Robin.)

tory movement, which lasts all through life: this movement is apparent, even some time after the death of the general organism (cessation of the circulation and innervation) these are the vibratile columnar epitheliums (Fig. 57).

The movements of the vibratile cilia of the cells are among the most curious phenomena presented by the epitheliums: the movement of the free cells, furnished in some cases with several cilia which assist them in locomotion, are of the same kind; we shall see further on that the spermatozoids are elements of this class; these elements become more numerous as we descend the scale, until, at length, we find them representing organisms which are endowed with a perfect individuality.

The cells having vibratile cilia are always cylindrical in the higher animals: in the mollusks and in beings of a still lower order, they appear under every possible form. It is remarkable that no epithelium with vibratile cilia has been observed among the articulata (insects). The cilia which spring from the base of the cells are generally fine and straight, but they are sometimes so bulky and their motion so extended, that the glittering waves which they produce on the surface of the mucus may be seen with the naked eye, as on the branchial lamellæ of the mollusks. On examining these movements with a powerful magnifying lens, we find that the cilia either bend in the shape of a hook, or perform