MUSCLE. 



515 



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 winch 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 aveolar 

 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 glutens 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. 



One result of the varied arrangement of the 

 tendinous fibres with regard to the muscular, 

 is the production of symmetry and beauty of 

 form; a second is convenient package; a third 

 is the adaptation of the particular muscle to the 

 kind and amount of exercise which is required 

 of it. Where a great mass of fibrous tissue runs 

 into a muscle, the number of fibres and their ob- 

 liquity is very much increased, while the length 

 of each is diminished; and, as a general result, 

 the power of such a muscle is augmented, 

 while the extent of its contractions is limited. 

 The same mass of contractile material may be 



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