the operation of that agent, is rent into shreds or segments, mostly annuiar or spiral, which cause the constrictions. In other cases the union of branches of the cells around a bundle may be the cause of the appearance.

The areolæ, or interstices of the areolar tissue, are intercommunicating cleft-like spaces between the bundles and laminæ. They are not present in the immature

Fig. 265.-ELASTIC FIBRES

FROM THE LIGAMENTUM
NUCHE OF THE ox,
SHOWING TRANSVERSE
MARKINGS ON THE FI-
BRES; HIGHLY MAGNI-

FIED. (E. A. S.)

tissue, in which the ground-substance is continuous through

out, but as the matrix becomes fibrillar the areolæ are formed, probably by the liquefaction of ground-substance.

In fibrous tissue the bundles of white filaments run parallel, cohering very intimately. They either run all in one direction as in long tendons, or intersect each other in different planes as in some aponeuroses, or they take various directions and decussate irregularly with each other as in the dura mater. And when they run parallel to each other, as in tendon, they do not keep separate throughout their length, but send off slips to join neighbouring bundles and receive the like in turn; so that successive cross-sections of a tendon or ligament present different figures of the sectional areas of the bundles. A sheath of dense areolar tissue covers the tendons and ligaments on the outside (fig. 264, a), and a variable amount of the same tissue (d, e) lies between the fasciculi into which the smaller bundles are grouped, separating them from one another, and also occurring, in greater amount, between the largest fasciculi (c). It is in these areolar tissue septa that the blood-vessels and lymphatics of a tendon or ligament run.

The surface of a tendon or of any other part consisting of this texture, appears marked across the direction of the fasciculi with alternate light and dark streaks which give it a peculiar aspect, not unlike that of a watered ribbon. This appearance is owing to the wavy course of the filaments, for when the light falls on them their bendings naturally give rise to alternate lights and shadows.

The fibrous and areolar tissues thus agreeing in their ultimate structure, it is not to be wondered at that sometimes the limits between the two should be ill-defined, and that the one should pass by inconspicuous gradations

into the other. Instances of such a transition may be seen in many of the fasciæ: these at certain parts consist of dense areolar tissue, but on being traced farther are seen gradually. to become fibrous; and fasciæ, which in one body are areolar in character, may be decidedly fibrous in another.

In the elastic tissue, there is a great proportionate development of the elastic fibres, the white bundles being relatively few and indistinct, but considerable variation is met with in the proportion of the two kinds of elements. The white bundles are, for the most part, disposed irregularly and course in different directions, as in areolar tissue; but, in some elastic ligaments, there are bundles of white fibres, which run as in an ordinary ligament parallel with one another, and from end to end of the structure. The elastic fibres in an elastic liga

ment, are collected into smaller and larger groups or bundles (fig. 266), which are separated from one another by septa of the white tissue, but the latter also penetrates between the individual elastic fibres of the group.

The cells or corpuscles of connective tissue.-Three kinds of cells may be distinguished in connective tissue, and these may provisionally be termed the flattened or lamellar, the granular, and the vacuolated or plasma-cells. They are all imbedded in the ground-substance, occupying the cell-spaces previously mentioned.

The flattened or lamellar cells (fig. 262, c, c and fig. 267), are often applied to the surfaces of the bundles of white fibres. Where three or more bundles come into

Fig. 267.-Two CONNECTIVE

TISSUE CORPUSCLES FROM
THE SUBCUTANEOUS CONNEC
TIVE TISSUE; HIGHLY MAG-

NIFIED. (E. A. S.)

The dark streak below 1, in the right hand corpuscle, is a lamella which happens to be projecting towards the observer and is seen in optical section.

apposition, the cells may extend between the several bundles, and they then consist of not one lamella, but of two, three, or more which fit in between the bundles, the body of the cell occupying the larger interstice. This is most marked in fibrous tissue, but is also seen in dense areolar tissue. The cells in some parts are united

Fig. 269.-EPITHELIOID AND RAMIFIED CELL-SPACES OF CONNECTIVE TISSUE. (E. A. S.)

340 DIAMETERS.

(From a preparation stained with nitrate of silver.)

The nuclei of the cells are indicated.

by their edges into patches, after the manner of an epithelium; in other cases, a union takes place by means of branching processes, so that the cells form a kind of network throughout the ground-substance, and a corresponding network is of course formed by the spaces in which the cells lie (figs. 268, 269). These flattened connective tissue corpuscles are composed of clear cell-substance, with but a few

minute granules scattered through it; they have a large oval nucleus with a fine intra-nuclear network and nucleoli.

In many membranous forms of connective tissue, the flattened cells form an epithelial-like covering to the surfaces of the membrane or membranes (fig. 270), and may even complete the latter by bridging over any gaps existing between the bundles of fibres forming the membrane. Such epithelioid tracts may be of considerable extent. It is often observable, that the cells at the margin of the patch have processes at their free border, which are connected with the ordinary scattered cells of the tissue (see fig. 269).

Granular cells.-Besides these flattened cells with fine granules, other corpuscles are met with in the connective tissue, which are more decidedly granular (fig. 262, g), having actual distinct, somewhat coarse, granules in their protoplasm. Although they were classed along with the vacuolated cells, next to be described, by

Fig. 270.-EPITHELIOID CELLS OF CONNECTIVE TISSUE.

FROM THE SURFACE OF AN APONEUROSIS

TREATED WITH NITRATE OF SILVER. HIGHLY MAGNIFIED. (E. A. S.)

Waldeyer, under the name of plasma-cells, they are perfectly distinct elements, and it will be better therefore to reserve the name "plasma-cells" for the vacuolated corpuscles, and to employ the name of granular cells for these, which are filled with obvious granules. The granular cells are especially abundant near the blood-vessels, and they also occur in large number in areolar tissue in which fat is to be deposited. The granules are of an albuminous nature; they stain deeply with eosin' and with many aniline dyes. These cells vary much in size and shape; many are spheroidal, but they may be branched or even flattened. Their nuclei are round or oval.

The plasma-cells (fig. 262, p. p.) are distinguished from the lamellar and granular cells by the extreme vacuolation of their cell-substance. The protoplasm between the vacuoles is clear, but may contain a few fine dark granules. The fluid which occupies the vacuoles is presumably of the nature of lymph or blood-plasma: it is less refracting than the substance of the protoplasm. It is not contained in the meshes of a spongioplasmic network but is in distinct vacuoles. These plasma-cells are frequently elongated and they may have short branching processes but they are seldom

Hence termed "eosinophile cells" by some authors. The same term has also been applied by Ehrlich to those leucocytes which have obvious granules, staining with eosin (see p. 213).

or never simply spheroidal like the granular cells. Cells of the same nature but with larger and more conspicuous vacuoles are met with in situations where capillary blood-vessels are about to be formed (see Development of Blood-vessels).

Cells are occasionally observed in areolar tissue in which nothing but a nucleus and a few scattered granules around it are visible (fig. 262, c). These are perhaps the remains of corpuscles which are in process of disintegration, but nothing is definitely known as to the removal and regeneration of the cells of connective tissue, nor as to the genetic connection, if any, existing between the several kinds of cells met with in the tissue. The three kinds of cells above described all belong to the so-called "fixed cells" of the connective tissue. The migratory cells," which are occasionally seen in areolar tissue, are identical with the pale blood- or lymph-corpuscles.

..

In areolar tissue all the varieties of connective tissue cells above described occur. They have no very definite arrangement. Both the cells and the spaces in which they lie may inter-communicate by their branches, and in this way it often happens. where the tissue is thicker, that the system of cells and cell-processes, and of corresponding canals, may effect a communication between the superficial and deeper parts of the tissue. The cells of areolar tissue are also connected with the flattened cells which line the smaller blood-vessels and lymphatics, and by means of this connection, and the continuity of the cell-spaces of the tissue, channels are provided for the flow of blood-plasma from the blood-vessels or towards the lymphatics. In addition to

this, no doubt some of the plasma or lymph may soak through the ground-substance, or find its way through the lacunar interstices (areolæ) of the tissue.

In fibrous tissue (tendon

and ligament), the cells, which

are often called "tendon- Fig. 271.-TENDON OF MOUSE'S TAIL, STAINED WITH LOGWOOD; cells," are all of the flattened

SHOWING CHAINS OF CELLS BETWEEN THE TENDON-BUNDLES.

175 DIAMETERS. (E. A. S.)

or lamellar variety. They follow the parallel arrangement of the fibre-bundles, and are disposed in rows or chains (fig. 271), which may be easily seen if a very fine tendon, such as those in the tail of the mouse or rat, or a portion only of a larger one is examined under the microscope, and a little dilute acetic acid is cautiously added. A peculiar shape is impressed upon these cells by the close packing of the tendon bundles, for although they may look quadrangular or oblong when the tendon is viewed longitudinally (figs. 271, 274), yet when it is cut across, they have a stellate appearance (figs. 264, 273), for like other flattened connective tissue cells, they send lamellar extensions into the interstices between the contiguous bundles, whilst the middle of each cell, containing the nucleus, lies in the angular space between three or more bundles. When the tendoncells are viewed longitudinally, any of the lamellar extensions, which are directed either towards or away from the observer, appear as lines on the surface of the cell (fig. 274). The same appearance is often seen upon the flattened cells of the denser forms of areolar tissue, where the cells have been squeezed in between three or more bundles.

Each tendon-cell consists of a delicate protoplasmic body, thicker at the centre and thinning off in the extensions, and containing a flattened, round or oval, clear nucleus, with an intranuclear network and several nucleoli. The ends of adjacent cells are in close apposition, and form, as before noticed, long chains of cells in the tendon, and the nucleus is generally so situated towards one end of the cell as to be

in close proximity to the nucleus of an adjacent cell; they thus present the appearance of being arranged in pairs (fig. 274). Here and there a third nucleus, with a small amount of protoplasm, may be seen interpolated between two such cells.

The lamellar extensions of the cells do not always end with an even line, but are themselves often prolonged into fine branches, which penetrate still further into the ground-substance which separates the fibre-bundles of the tendon from one another.

In the pure elastic tissue, such as that which constitutes the ligamentum nucha

Fig. 272.-CELL-SPACES OF TENDON OF MOUSE'S TAIL, BROUGHT INTO VIEW BY TREATMENT WITH NITRATE OF SILVER. 175 DIAMETERS. (E. A. S.)

(E. A. S.)

175 DIAMETERS.

The flattened processes of the tendon-cells (which are stained deeply by logwood) appear in section as lines, frequently coming off at right angles from the body of the cell. The bundles of fibres are not represented; they are very irregular, and but incompletely separated by the cell-processes.

in animals and the ligamenta subflava in man, it is generally stated that cells are altogether absent. Schwalbe finds on the contrary numerous flat connective tissue

Fig. 274.-EIGHT CELLS FROM THE SAME TENDON AS REPRESENTED IN FIG. 271. MAGNIFIED 425 DIAMETERS. (E. A. S.)

The nuclei, with their numerous nucleoli, were deeply coloured by the logwood. The dark lines on the surface of the cells are the optical sections of lamellar extensions directed towards or away from the observer.

cells scattered in the ground-substance which lies between the elastic fibres; the cells being often in close apposition with the elastic fibres, but never in continuity with them, as has been described by some authors.

Vessels and nerves.-Blood-vessels, lymphatics, and nerves are everywhere conveyed in the areolar tissue to the places where they are to be distributed, but very few blood-capillaries are destined for the tissue itself, although abundant lymphatic networks are present in many parts; especially in the subcutaneous, subserous and submucous tissues. It is uncertain whether nerves terminate in the areolar tissue. It may be cut in a living animal apparently without giving pain, except when the instrument meets with those branches of nerves which traverse the tissue on their way to other parts.

The fibrous tissue receives blood-vessels, but in general they are inconsiderable both in number and size compared with the mass of tissue to which they belong. In tendons and ligaments with longitudinal fasciculi, the chief branches of the vessels run parallel with and between the larger fasciculi, and, sending communicating branches across them, eventually forming a very open network with large oblong meshes. Some fibrous membranes, as the periosteum and dura mater, are much