Of the mode of aggregation of the elemen tary fibres.—The two kinds of fibre whose structure has now been described, are aggr gated into masses of very various shape bulk, and supplied with areolar tissue, and nerves, so as together to form the termed muscles. But if we trace these orga downwards through the animal scale, we co to examples in which solitary fibres exist wi out any such appendages, and yet evident performing the office of, and truly constautin a perfect muscle. And even many fibres are found, so far smaller than the usual dimen sions as to consist of only one or two linear series of sarcous elements, and these perhaps only visibly present near the centre of the fibre, where developement is most advanced, and the contractile energy greatest. In such minute and simple forms we may perceive a transition from the striped towards the unstriped fibre, the transverse lines being often irregular, broken, or faintly marked. And we may also discern a clue to the meaning of the structural condition which is found in the complicated muscles of the higher animals. The essential contractile material is the fibre, and its mass is accurately proportioned to the power de manded. If this is below that of a single elementary fibre, the fibre is reduced in pro portion ; if more is required than one fibre can supply, the size of this is not increased but its number multiplied. The point at which an increase in number supersedes one in size, is that which has been already stated to be the average bulk of the fibre. This differs in the different classes of animals, and corresponds with the demand there may be in each class for vascular and nervous supply. For by the very constitution of the contractile materal, it can receive neither vessels nor nerves into its interior substance, and therefore it must be itself subdivided further and further in pro portion to the amount of these which are to be in contact with its surface.
In the compound organs termed muscles, the fibres are usually disposed in parallel sets of 10, 20, 30, or more, surrounded and held together by a delicate areolar tissue, which penetrates more or less among the individual fibres, but does not necessarily invest each one of them from end to end, as it is frequently described to do. Where the fibres are not very large, it is often difficult to discern any areolar tissue at all in connexion with them. These first sets admit of considerable motion on one another, in consequence of the looseness of their areolar sheath. Like the elementary fibres themselves their figure is polygonal, for they in their turn are arranged (if the muscle be large enough) into secondary sets, and are flattened by being pressed together. These again are aggregated into tertiary sets, and these into still larger ones, according to the size of the particular organ. All these sets partake of the polygonal figure of the elemen tary fibres; except the portion that forms a part of the general exterior of the muscle, which is usually more or less rounded. As the packets of fibres are larger, so their angles are more rounded, and their surface covered with a more abundant areolar sheath, and they approach, in fact, to the condition of a perfect muscle, which is itself included in an envelope of areolar tissue. Their angles are thus rounded
in consequence of the greater quantity of areo lar tissue, and of the larger size of the vessels and nerves that occupy their intervals. For the same reason the elementary fibres themselves are less sharply angular, when very small, as in Birds, because the vessels accompanying them are proportionally more abundant, and occupy more space in their intervals.
The arrangement of the elementary fibres into these packets has received more attention than its importance deserves, and anatomists have endeavoured to affix definite names (fasci culi, lacerti, &c.) to certain sizes of them. But no division of this kind is to be found in nature. It may be safely said that packets of every possible bulk exist from the simple set of two or three fibres, to those of many thou sands, these last being subdivided with the greatest irregularity. And it is only in muscles possessed of some thickness that any such package in sets is to be met with. The ab dominal plane muscles, which contain such an arrangement in the larger animals, are, in the smallest, composed of a single unbroken layer of elementary fibres. A similar diversity exists between muscles of different size and shape in the same animal. In the gluteus maximus, which is liable to pressure and change of position from its peculiar situation, the fibres are made up into lacerti, about one-quarter of an inch thick, surrounded with a dense areolar sheath, and attached loosely to one another ; while the glutei situated underneath are, like the psoas, unprovided with such dense septa of areolar tissue, and seem more uniform throughout.
From these and many other considerations which might be adduced, it may be clearly seen that the mere aggregation of the elemen tary fibres in a muscle into larger or smaller sets, is determined solely by its own peculiar cir cumstances and exigencies, and is not of a nature to demand particular description in so general an account as the present.
The direction of the elementary fibres of vo luntary muscles is usually straight, between their points of attachment, which are always some form of the fibrous tissue. This tissue may be so arranged as that the sets of mus cular fibres passing from it, may be either parallel or oblique to one another, but the fibres forming any one set are generally placed in a parallel series. If the fibrous tissue form a laminar expansion on the surface of a muscle, the muscular fibres pass off from it obliquely, either to a similar expansion on the opposite surface or to a tendon. If they arise from an extensive surface of bone (i. e. of periosteum) they conduct themselves in a similar fashion, and also if they pass from a line of tendon or of bone. In all these cases the muscle may be styled penniform. If a thread or sheet of fibrous tissue dip into the interior of a muscle, it gives origin to the muscular fibres on both sides, and they diverge from it obliquely : such a muscle is styled doubly penniform. When several such sheets enter the muscle at both extremities, and give attachment to the fibres obliquely placed in the intervals, the muscle is styled compound penniform, as the deltoid.