On this view we may regard serous mem branes as forming one of a group of tissues. A further analysis of this group shows it to be composed of several members, separated from each other by differences, in which we may recognise a progressive, though some what interrupted, series of gradations. These differences we shall now proceed rapidly to trace.
Two important microscopical elements pervade all these structures, and will there fore demand some attention. These are the white and yellow fibrous tissues.
The white fibrous tissue (fig.395. a) consists of bands or bundles of a very variable width, which, unless artificially stretched, take a sinuous or wavy course ; and, at distant in tervals, include cell-nuclei in their substance. They are marked with strim, which take the direction of their length, and, by their mutual proximity, give a fibrous or fibrillated ap pearance to the whole mass. But these markings are not exactly parallel to the borders of the band ; and since the tissue, though easily divided longitudinally to almost any degree of rainuteness, cannot be split up into uniform and definite fibrils of a diameter corresponding with the transverse width which intervenes between one of these strim and another ; and since it is also swelled up into one shapeless and semitransparent mass by the action of acetic acid ; it seems hic,..hly probable that they are limited to the surface of the bundle, or its immediate neighbourhood. At any rate, they do not sufficiently divide the mass to give it a filamentous constitution, or to render it " fibrous " in the true sense of the word.
The yellow fibrous tissue (fig. 395. b) is contrasted with the preceding form, not only by its colour, but equally by its minute struc ture and properties. It consists of separate fibres, the size of which varies considerably in different parts, and, in a lesser degree, in any one specimen. They are exceedingly disposed to curl up, often assuming almost a spiral form ; and are rendered very distinctly visible by the dark margin which their great refracting power gives them. Their branchings are generally dichotomous, and the processes thus given off' are of a size which nearly equals that of the original stem ; and they may often be traced to their union with neighbouring ones, so as to form a kind of trellis-work.
The first form exists alone in tendons, ligaments, and the stronger fasciae latx ; its inextensibility and strength admirably adapting it to the use of mere passive resistance to an external force. The second is highly elastic, whence it is often termed" the elastic tissue :" it is chiefly found where, along with a certain amount of yielding, is also required a complete restoration of the previous state without any further expenditure of muscular force, the long duration of an action often rendering it advantageous to avoid the fatigue which the constant exercise ofvolition and muscle would imply. And as these conditions are much
rarer than the simply mechanical wants which the preceding form is destined to supply, so also is the tissue which fulfils theib, being found separately in but a few parts of the body; viz. in the ligamenta subflava, and in certain portions of the vocal and respiratory apparatus. Here it is in sparing quantity ; but in the vast ligamentum nuchae, which suspends the ponderous heads of the horned graminivora, the uses of the tissue are ex-. emplified in a very striking manner.
Chemically, they are distinguished by the white fibrous tissue containing much gelatine, or rather yielding it by boiling ; while, froin the yellow, none can be obtained. They are both little disposed to putrefaction, and retain their peculiar physical properties almost un impaired by time.* A mixture of these two elements consti tutes the areolar tissue, which enters so largely into the formation of almost all the organs. The bands of the one and the fibres of the other are closely interwoven, although with out mutual continuity ; each giving off branches which again unite with the other neighbouring subdivisions of the same kind, so as to form a complicated interlacement of the two net works. This arrangement results in an innu merable series of meshes, which everywhere communicate with those in their immediate proximity, and the size and shape of which varies within very wide limits. And these limits are frequently still further extended, since the separation of some of these micro scopic meshes, and the approximation and condensation of others, gives rise to the for mation of a secondary net-work, which is visible to the naked eye, and which, though still open in every direction, possesses, espe cially in inflated and dried preparations, an appearance sufficiently resembling that of cells to remind one of the name formerly applied to this structure, which was called, as if itor)v, "the cellular tissue." The proportion in which these two consti tuents are mixed varies greatly in the areolar tissue of different parts of the body ; the pre. ponderance of one over the other following that of the conditions which were previously stated to regulate their separate presence. Thus, the likelihood of its frequent and great distention is often a requisition of increased elasticity, and is then accompanied by an increased proportion of the yellow element.