508 



MUSCLE. 



The most deeply coloured muscle I have seen 

 was the great pectoral muscle of the Teal 

 (Querquedula ciecca), killed after migration. 

 In Mammalia the colour is ordinarily red, 

 being deeper in theCarnivora than in the vege- 

 table feeders. Among the domestic animals 

 many varieties exist, which need not be spe- 

 cially enumerated. A considerable part of the 

 colouring matter is extracted by repeated wash- 

 ing of a muscle, which then becomes pale, but 

 not quite colourless ; some part of the loss of 

 colour here sustained is doubtless owing to the 

 solution of the haematosine of the blood con- 

 tained in it. A muscle, if hypertrophied, 

 grows redder, and vice versa ; and probably 

 the practice of bleeding calves some days 

 before they are killed, makes their flesh more 

 pale and tender, by causing the absorption of a 

 portion of the proper colouring matter of the 

 fibres, as well as by abstracting the blood 

 circulating among them. 



5. Internal structure. Though the elemen- 

 tary fibres of all animals are visible to the 

 naked eye, and in some animals, as the Skate 

 (Itaia Batus), are often as thick as a small pin, 

 nothing of their internal organization can be 

 distinguished without the aid of a powerful 

 lens. There is indeed, in certain lights, a 

 splendid pearly iridescence, resulting from 

 the arrangement of their structure, and quite 

 characteristic among the soft tissues; but this is 

 not explained till a high power of the micro- 

 scope is brought to bear upon the fibres. They 

 are then seen, when viewed on the side, to be 

 marked by innumerable alternate light and dark 

 lines, whose delicacy and regularity nothing can 

 surpass, and which take a parallel direction 

 across them; and if the focus be altered so as 

 to penetrate the fibre, they are found to be pre- 

 sent within it just as on its surface, thus differ- 

 ing from those on the tracheae of insects, which 

 exist only at the surface. At the extreme border 

 of the fibre the light lines are sometimes seen to 

 project a trifling degree more than the dark 

 ones, thus giving a slight scallop, or regular 

 indentation, to the edge. If often happens, in 

 tearing the fibres roughly with needles before 

 examination, that they crack across, or give way 

 entirely, along one or several of these dark lines, 

 the line of fracture or cleavage running more or 

 less completely through the fibre in a plane at 

 right angles with its axis; and occasionally two 

 or more of such complete cleavages will occur 

 close together, the result of which is the separa- 

 tion of so many plates or discs (fig. 287, B), of 

 which the light lines at the surface are the edges, 

 and the corresponding light lines seen within 

 are what may be termed the focal sections. 

 Thus it is evident that there is a tendency in 

 the mass of the Jibre to separate, when torn or 

 pulled ajter death, along the transverse planes, of 

 which the dark transverse stripes are the edges. 

 When such a separation takes place, a series 

 of discs result, but to say that the fibre is a 

 mere pile of discs is incorrect, for the discs are 

 only formed by its disintegration. Neverthe- 

 less they are marked out, and their number 

 and form are imprinted, in the very structure 

 of the fibre, in its perfect state. (Figs. 287 

 and 288.) 



Fragments of striped elementary fibres, shewing a 

 cleavage in opposite directions, magnified 300 diam. 



A, longitudinal cleavage. 



At a the longitudinal and transverse lines are 

 both seen. Some longitudinal lines are darker and 

 wider than ihe rest, and are not continuous from 

 end to end. 



b, primitive fibrillae, separated from one another 

 by violence at the broken end of the fibre, and 

 marked by transverse lines equal in width to those 

 at a. 



c represents two appearances commonly presented 

 by the separated single fibrillae. On the upper one 

 the borders and transverse lines are all perfectly 

 rectilinear, and the included spaces perfectly rect- 

 angular. In the lower the borders are scalloped, 

 the spaces bead-like. When most distinct and de- 

 finite, the fibrilla presents the former of these ap- 

 pearances. 



B, transverse cleavage. The longitudinal lines 

 are scarcely visible. 



a, incomplete fracture following the opposite 

 surfaces of a disc, which stretches across the inter- 

 val and retains the two fragments in connexion. 

 The edge and surface of this disc are seen to be 

 minutely granular, the granules corresponding in 

 size to the thickness of the disc and to the distance 

 between the faint longitudinal lines. 



b, another disc nearly detached. 



But again, it always happens that longitu- 

 dinal lines, more or less continuous and pa- 

 rallel, according to the integrity of the fibre 

 and the strength and distinctness of the trans- 

 verse lines, are also to be discerned ; and like 

 the transverse ones, not on the surface only, but 

 throughout the whole of its interior. And it is 

 found that there is a remarkable proneness in 

 the fibre to split in the direction indicated by 

 these lines also; by which splitting it is resolved 

 into a great number of Jibrilla. These fibrillas, 

 like the discs, do not exist as such in the fibre, 

 and to obtain them its structure must be neces- 

 sarily broken up to a certain extent, for the 

 union which naturally subsists between these 

 parts must be destroyed. It is therefore most 

 correct to say that there is an indication in the 

 entire state of the fibre of a longitudinal ar- 

 rangement of its parts, occasioning a cleavage 

 in that direction on the application of violence. 

 (F'g. 287.) 



Sometimes the fibre will split into discs 

 only, more often into fibrillae only, but there 

 are always present in it the transverse and the 

 longitudinal lines which mark the two cleav- 

 ages. It is the most common to find a crack 

 or fracture taking both directions irregularly, 

 running partly in the transverse dark lines, 



