Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/905

Rh TEXTUHES.] ANATOMY and assume a fusiform, caudate, or stellate shape; and, subsequently a delicate fibrillated structure appears between them, which assumes the characters of the bundles of white fibrous tissue, and by separating the cells from each other forms the fibrous intercellular matrix. It has been much disputed whether these white fibres take their rise immediately from the peripheral portion of the cells by a direct differentiation of their protoplasm, or whether this protoplasm is not in the first instance converted into a homogeneous matrix in which the fibrous differentiation then occurs. There can be no doubt that the fibres are formed by a metamorphosis of the protoplasm of the cells ; whether the metamorphosis takes place directly, or through the intermediate stage of a homogeneous matrix, is a secondary question, and in all probability both modes of conversion take place at different times and in different localities. As the differentiation into fibres progresses, the tissue becomes firmer and tougher, and the proportion of the cellular to the fibrous element diminishes. Hence, say in a young tendon, the rows of connective tissue cells are not only closer together, but are much more readily seen than in an adult tendon, in which the increased production of fibres obscures the cellular element. The mode of origin of the yellow elastic fibres has also been much discussed. At one time it was believed that they were derived from nuclei, and on this supposition they were named nuclear fibres. But from more recent observa tions there is reason to believe that they are produced, like the white fibres, by a special differentiation of the proto plasm of the embryonic cells, or of a homogeneous matrix derived from that protoplasm. In such localities as the ligameutum nuchse, where the fibres are both large and numerous, the whole of the cell protoplasm appears to become converted into elastic tissue. In tendons, and those parts where these fibres are slender and scanty, and coil round the bundles of white fibrous tissue, they apparently arise from a differentiation of the protoplasm on the sur face only of the formative embryonic cells. ADIPOSE TISSUE. The adipose or fatty tissue varies in its amount in different individuals. It is especially found in the marrow of the bones; as a layer beneath the skin, differing in thickness in different individuals; and collected in the cavity of the abdomen in the folds of peritoneum, known as the mesentery and omenta, in which, and indeed in the other localities where it occurs, it is intimately associated with the connective tissue. It consists of cells, which vary in size from s^-g-th to -g^th inch, usually ovoid or spherical in form, though when collected into masses they may be laterally compressed. These cells are sometimes isolated, though most usually arranged in rows or clusters to form lobules of fat. The number of cells in a given lobule varies with the size of the lobule. The distinctive contents of these cells is a minute drop of oil, which, when examined by transmitted light, presents a bright appearance; but when seen by reflected light, looks, as &quot;^^^u&quot;^^* Monro primus described it long ago, like a cluster of pearls. Each fat cell possesses a distinct wall, as can be readily demonstrated by digesting these cells in ether, when the oil is dissolved out and the membranous wall remains. The nucleus of the fat cell is more difficult to demonstrate, and when seen is found attached to the inner surface of the cell wall. In the fat of old persons, and in specimens of this tissue which have been removed from the body for a length of time, a stellate group of acicular crystals is not unfrequently to be seen in the interior of the cell, which consists either of margarin or margaric acid, one of the constituents of human fat. The lobules of fat cells are included between bundles of the areolar variety of connective tissue, which form their supporting framework. But in addition, they are more or less per fectly surrounded by a network of capillary vessels, which not only serves to convey to them blood for their nutrition, but aids in retaining them in position. The close anatomical relation between the adipose and the connective tissue points to a genetic relationship between them. It has now been ascertained that the first stage in the formation of a fat cell consists in the appearance of extremely minute drops of oil in the protoplasm of the con nective tissue corpuscles of the part ; as these run together larger drops are produced, a cell wall at the same timo differentiates from the peripheral part of the protoplasm, and as the cell becomes distended with oil, by the conver sion into fat of its substance, it swells out into a spherical or ovoid cell. Klein has recently shown that the fatty tissue of the omentum and mesentery is formed by the production of oil drops within the branched cells, which form the reticular tissue that supports the lymphoid cells found so abundantly between these folds of peritoneum. PIGMENTARY TISSUE. In some parts of the body a Pigment, yellow, brown, or black pigment is found in the interior of cells, which gives to the tissue and organ a characteristic colour. In the coloured races of mankind, and in certain parts of the body of the white races, pigment is produced in the cells of the cuticle or epidermis, more especially in the cells of the deeper strata or rete Malpighi. In the con nective tissue corpuscles, also, more especially in the dermis of fish, amphibia, and reptiles, pigment is found in con siderable abundance. The choroid coat of the eyeball owes its dark brown or black colour to the presence of pigment in the interior of the cells. The pigment cells of the choroid are usually polyhedrons, 5 or 6-sided, and are arranged to form a mosaic pattern. In the centre is a nu- cleus, and the cell substance is occupied by numbers of minute brown granules. In the connective tissue on the outer surface of the choroid, the pigment is contained in stellate cells. In the skin of fishes and amphibia, the stellate pigment cells branch and subdivide so as to form highly complex patterns, and the cells are crowd ed with brown or yellow granules. The production of pigment, either in the in terior of epidermal cells, in the poly hedral cells of the choroid, or in the stellate connective tissue corpuscles, is owing to a special metamorphosis or dif- FJ{J 44 _ stcllate pig. ferentiation of the protoplasm substance mcnt ceils from the Of these Cells. skin of a codflsb. CAKTILAGINOUS TISSUE. By the term cartilage, or Cartilage, cartilaginous tissue, is meant a group of tissues which, though usually found in the form of plates or bars, yet differ in various aspects from each other, both in naked eye and microscopic characters. They agree, however, in forming solid textures, opaque when seen in mass, but, in thin slices, translucent, pearly, or bluish white, firm in consistence, but easily cut with a knife, endowed with considerable elasticity, and yielding chondrine on boiling. Cartilage is of greater importance in the foetus, and in the immature condition of the body than in the adult, for in early life the bones are in a great measure formed of it. As development and growth proceed, a considerable pro portion of the cartilage becomes converted into bone, and is called, therefore, temporary cartilage, whilst the remain ing portion continues as cartilage throughout life, and is chcr oidal pigment cells.