otherwise removed, as by a short treatment with dilute alkali, the fibres of the reticulum come clearly into view (figs. 280, 281). The true structure of this tissue

was first pointed out by Bizzozero.

In many situations the meshes of the retiform tissue are occupied by numerous corpuscles which closely resemble the pale blood- or lymph-corpuscles, but have a

tr.

Fig. 280.-RETICULUM FROM THE MEDULLARY PART OF A LYMPHATIC GLAND. (E. A. S.) tr, end of a trabecula of fibrous tissue; r, r, open reticulum of the lymph-path, continuous with the fibrils of the trabecula; r', r', denser reticulum of the medullary lymphoid cords. The cells of the tissue are not represented, the figure being taken from a preparation in which only the connective tissue fibrils and the reticulum are stained.

relatively larger nucleus, and less protoplasm than those. They are known as lymphoid cells, and the tissue containing them is termed lymphoid or adenoid tissue. This tissue is found composing the greater part of the lymphatic glands,

tr, trabecula; r, reticulum. and other structures allied to them, such as the solitary and agminated glands of the intestine, and the similar structures in the tonsils and elsewhere. Moreover, the alimentary mucous membrane is in some parts composed of the same tissue, and it occurs also in other mucous membranes and, in the form either of elongated tracts or of isolated nodules, in many parts of the serous membranes. In the spleen, the interstices of the retiform tissue are for the most part occupied by blood, instead of by lymph as elsewhere. In organs into the construction of which this tissue enters, it serves as a supporting framework to those parts of the organ int. which connective tissue of the ordinary kind does not penetrate.

DEVELOPMENT OF CONNECTIVE TISSUE.

Those parts of the early embryo in which connective tissue is subsequently to be developed, are at first composed entirely of embryonic cells, to all appearance similar to those which constitute the mesoblastic layer generally. It is, however, believed by many authorities that in their origin the cells which form the connective

tissues and the vessels are esentially different from those which produce the other tissues, and that they have wandered in between the other cells of the mesoblast from the peripheral part of the blastoderm (parablast-cells of His, mesenchyme cells of Hertwig. See Embryology, pp. 25-27).

The mesoblast cells which are concerned in the formation of the connective tissues are at first rounded in shape, and loosely packed, and exhibit amoeboid movements when examined on the warm stage. Subsequently they become irregularly ramified and tend to unite with one another so as to form a kind of cellnetwork with open interstices. These interstices are at first occupied by an albuminous fluid which later acquires a mucous or muco-albuminous character and becomes more consistent it may now be spoken of as the ground-substance or matrix.

In this ground-substance fibres become developed of the two kinds, white and elastic, but the manner in which they are formed is by no means clear; and two distinct and opposed views are held by histologists upon the subject. According to the one view, the bundles of white fibrils are produced by a direct conversion of the protoplasm of some of the cells, the others remaining as the permanent connective tissue corpuscles; or the permanent corpuscles represent embryonic cells, layers of whose protoplasm have been successively converted into fibrillar tissue, the cells, meanwhile, after each such conversion, growing again to their original size,

cesses of neighbouring cells, and undergoing a chemical transformation, produce the networks of elastic fibres. According to the other view the fibres, both white and elastic, are formed by a deposit in the intercellular substance, and not by a direct change of the protoplasm of the cells, with which indeed they are not connected; although it is not excluded that the deposition may in some way or other be influenced, or even caused by the pre-existing cells.

In favour of the former view is the fact that in young connective tissue there are sometimes to be seen long cells with fibrillated protoplasm which might be regarded as in process of conversion into bundles of white fibrils. And various authors have described an apparent continuity both in young and in developed connective tissue of the elastic fibres with the cells of the tissue, or even with their nuclei.

In favour of the latter view may be instanced the appearance of the jelly-like connective tissue of the early embryo in which the fibres of both kinds can be seen coursing through the jelly-like intercellular substance, apart entirely from the cells. In the case of the elastic fibres, these, as shown by Ranvier, appear in the form of rows of granules or globules, which subsequently become fused together end to end, and are not at any time continuous with cells (fig. 282). To form an elastic membrane, in place of being arranged in lines the globules are deposited in small patches,

and by their fusion the membrane is formed (p). In elastic cartilage the granules first make their appearance, it is true, in the immediate neighbourhood of the cartilage-cells; but although this renders it probable that the deposition of the granules is influenced by the cells, it does not prove that they are formed by a direct conversion of the cell-protoplasm. Indeed, the subsequent extension of the fibres into those parts of the matrix which were previously clear of them (a process which can be easily followed in the arytenoid cartilage of the calf), and in which no such direct conversion of cell protoplasm seems possible, is a strong argument in favour of the deposition hypothesis.

The view which supposes that a direct conversion of the protoplasm of the connective tissue cells takes place into fibres, both white and elastic, has of late years been widely adopted, but it seems to rest largely upon a desire to interpret the facts in accordance with the conception (originally formulated by Beale and M. Schultze), according to which every part of an organised body consists either of protoplasm (formative matter), or of material which has been protoplasm (formed material); the idea of a deposition or change occurring outside the cells in the intercellular substance being excluded. It is, however, not difficult to show that a formation of fibres may occur in the animal organism without a direct transformation of protoplasm, although the materials for such formation may be furnished by cells. Thus in those colenterates in which a low form of connective tissue first makes its appearance, this is distinguished by a total absence of cellular elements, a ground-substance only being developed and fibres becoming formed in it. Again, the fibres of the shell-membrane of the bird's egg are certainly not formed by the direct conversion of the protoplasm of the cells which line the oviduct, although they are formed in matter secreted by those cells, and it is through their agency that the deposit occurs in a fibrous form.

In the formation of retiform tissue the ground-substance appears to become entirely liquefied except where it enters into the composition of the reticulum, and the cells of the tissue become applied to the anastomosing fibril bundles, and by their union constitute a network of branched cells enveloping the network of fibrils.

r, ramified cells intercommunicating by their branches; l, a row of leucocytes or migratory cells; f, f, fibres coursing through the ground-substance.

In lymphoid tissue the meshes become occupied by lymph corpuscles which may originally have come from the blood or lymphatic vessels, but afterwards multiply by cell-division.

The jelly-like connective tissue of the early embryo persists in the umbilical cord until birth as the so-called jelly of Wharton (fig. 283). Elsewhere it has largely lost its jelly-like character in consequence of the development of fibres in the ground-substance, but the amount to which they are developed varies greatly in

different animals, and in the connective tissues of different parts. In the vitreous humour of the eye no fibres are developed, and the cells become for the most part either atrophied or much modified, and remain relatively few in number. The so-called jelly-like connective tissue which is thus produced consists therefore almost entirely of ground-substance.

Connective tissue appears to be readily regenerated, although the new cicatricial tissue which is formed in place of that which has been removed by the knife or by disease, is not always obviously of the same character, either as regards its cells or fibres, as the tissue it replaces.

As to the mode of its regeneration there is still some uncertainty. It was believed by Cohnheim, whose views have been supported by the experiments of Ziegler, that the new tissue was formed by the leucocytes (lymph-corpuscles) of the granulation tissue which first appears in the wound. But it is affirmed on the other hand by other observers that the leucocytes, although unquestionably the precursors of the newly-forming tissue, do not take any direct part in its formation, but are gradually replaced by plasma-cells of the surrounding tissue which also wander into the space within which the new tissue is to become formed. Here they produce the cicatricial tissue, either by immediate transformation of their protoplasm into fibrils, or by an extracellular formation in the adjacent ground-substance.

RECENT LITERATURE.

Altmann, Ueber die Fettumsetzungen im Organismus, Arch. f. Aṇat. u. Phys., Anat. Abth., Suppl. Heft, 1889.

Bambeke, C., v., De l'origine des tissus de substance conjonctive, Annales de la société Belge de microscopie, xii., 1889.

Beltzow, A., Untersuchungen über Entwickelung und Regeneration der Sehnen, Archiv f. mikr. Anat., Bd. 22, 1883.

Bobritzky, C., Zur Kenntniss des Baues, der Entwicklung und der regressiven Metamorphose der Fettzellen, Medicin. Centralb. No. 43. 1885.

Ehrmann, S., Ueber Fettgewebsbildung aus dem als Winterschlafsdrüse bezeichneten Fettorgan, Sitzungsber. d. Wiener Akad., Bd. 87, 1883.

Ewald, A., Zur Histologie u. Chemie der elastischen Fasern u. des Bindegewebes, Zeitschr. f. Biologie., viii., 1889.

Frommann, C., Structur der Fettzellen und ihre Membran, Jenaische Zeitschrift für Naturwissenschaften, xvii., 1884.

Heller, J., Die Histogenese der elastischen Fasern im Netzknorpel und Ligamentum nuchae, Dissert., Berlin, 1887.

Jakowski, M., Ein Beitrag zur Lehre von der Entwicklung des Fettgewebes, Festschrift zur Feier des 25 jährigen Jubiläums von Prof. Hoyer, Warschau, 1884.

Kölliker, A., Zur Entwicklung des Fettgewebes, Anatom. Anzeiger, No. 8, 1886.

Kuskow, N., Beiträge zur Kenntniss der Entwicklung des elastischen Gewebes im Ligamentum nuchae und im Netzknorpel, Archiv f. mikrosk. Anatomie, Bd. 30, 1887.

Lwoff, Ueber die Entwickelung der Fibrillen des Bindegewebes, Wiener Sitzungsb., 98, 1889. Mall, F., Reticulated and yellow elastic tissues, Anat. Anzeiger, iii. Jahrg., No. 14, 1888. Martinotti, De la réaction des fibres elastiques avec l'emploi du nitrate d'argent, Arch. ital. de biologie, xi., 1889.

Metzner, R., Ue. die Beziehungen der Granula zum Fetiansutz, Arch. f. Anat. u. Phys., Anat. Abth., 1890.

Nordmann, O., Beiträge zur Kenntniss und namentlich zur Färkung der Mastzellen, Internat. Monatsschr. f. Anat. u. Histol. ii., 1885.

Ognew, J., Zur Frage von der morphologischen Bedeutung des fibrillären Bindegewebes, Archiv. 1. Anat. u. Physiol., Anat. Abth., 1885.

Poljakow, Ueber eine neue Sorte Zellen, ans welcher die fettbildenen Organe im Bindegewebe bestehen, Russian: Abstr. in Hermann and Schwalbe's Jahresbericht, 1888.

Randnitz, R. W., Beitrag zur Kenntniss der im Bindegewebe vorkommenden Zellen, Archiv. f. mikr. Anat., Bd. 22, 1883.

Ranvier, L., Les éléments et les tissus du système conjonctif, Journal de micrographie, 1889,

1890.

Sherrington, C. S., and Ballance, C. A., On formation of scar tissue, Journal of Physiology, x., 1889.

CARTILAGE.

This is the well-known substance commonly called "gristle." The following are its more obvious characters. When in mass, it is opaque and of a pearly or bluish white colour, in some varieties yellow; but in thin slices it is translucent. Although it can be easily cut with a sharp knife, it is nevertheless of very firm consistence, but at the same time highly elastic, so that it readily yields to pressure or torsion, and immediately recovers its original shape when the constraining force is withdrawn. By reason of these mechanical properties, it serves important purposes in the construction of some parts of the body.

In the early embryo the skeleton is, in great part, cartilaginous; but the cartilage forming its different pieces, which have the outward form of the future bones, in due time undergoes ossification or gives place to bone, in the greater part of its extent at least, and hence this variety of cartilage is named "temporary."

Of the permanent cartilages a great many are in immediate connection with bone, and may be still said to form part of the skeleton. The chief of these are the articular and the costal cartilages; the former cover the ends or surfaces of bones in the joints, and afford these harder parts a thick springy coating, which breaks the force of concussion and gives ease to their motions; the costal or rib-cartilages form a considerable part of the solid framework of the thorax, and impart elasticity to its walls. Other permanent cartilages enter into the formation of the external ear, the nose, the Eustachian tube, the larynx, and the windpipe. They strengthen the substance of these parts without undue rigidity; maintaining their shape, keeping open the passages through them where such exist, and giving attachment to moving muscles and connecting ligaments.

Cartilages, except those of the joints, are covered externally with a moderately vascular fibrous membrane named the perichondrium.

When a very thin slice of cartilage is examined with the microscope, it is seen to consist of nucleated cells, disseminated in a solid mass or matrix (fig. 285). The matrix is sometimes transparent, and to all appearance homogeneous; sometimes dim and very faintly granular, like ground glass: both these conditions occur in hyaline cartilage, which may be regarded as the most typical form of the tissue. Two varieties exist in which the matrix is pervaded to a greater or less extent by fibres. In the one, named elastic or yellow fibro-cartilage, the fibres are similar to those of elastic tissue; in the other, named white fibro-cartilage, they are of the white kind as in ordinary ligament.

HYALINE CARTILAGE.

Structure. In hyaline cartilage the matrix, as just stated, is uniform, and, when examined fresh, usually appears free from fibres. Like the ground-substance or matrix of connective tissue, it becomes stained brown by nitrate of silver and subsequent exposure to the light. The cells consist of a rounded, oval, or bluntly angular cell-body of translucent protoplasm, embedded in which are fine curvilinear interlacing filaments and minute granules (fig. 284), with a round nucleus, which is either clear with one or more nucleoli, or, more commonly, is occupied by a network of chromoplasm, which produces under a low power of the microscope a granular effect. The cell-body lies in a cavity of the matrix, which, in its natural condition, it entirely fills. This cavity is bounded and inclosed by a transparent capsule, which is seldom obvious to the eye, for it coheres intimately with the surrounding matrix, with which it agrees in nature, and cannot usually be distinguished without the aid of re-agents.