There is perhaps no one line of inquiry in the whole range of minute anatomy so beset with difficulties and sources of error, and therefore so much demanding a cautious study and sagacious discrimination between conflict ing appearances, as this of the structure of the striped fibre. The following description is substantially the same as that published by me in the Philosophical Transactions, 1840, and which all my subsequent observations have tended to confirm. To that paper I would venture to refer those who may desire to enter at greater length upon the grounds of the view here summarily given.
1. Length.—This varies exceedingly in dif ferent muscles. The sartorius, the longest in the body, often surpasses two feet in length, and the individual fibres are as long, extending in parallel bundles from end to end. In many others they do not exceed a quarter of an inch ; thus their greatest variety is presented in their length.
2. Thickness.—This should be examined in the uncontracted state of the fibre, which for this purpose should be removed from the body after all contractility has departed. I have elsewhere* given a table of numerous compa rative measurements in various animals, and subjoin the following abstract :— Diameter of the elementary fibres of striped muscle infractions of an English inch. From to Human average of males females Other Mammalia average 3BI Birds itlas Shy Ut7 Reptiles Ika 1h, ,, Fish 7-13 Insects tea, Zig I believe that the average diameter of the fibres in the human female is upwards of a fourth less than in the male, and that the ave rage of both together is greater than that of other Mammalia; but a more extensive exami nation is requisite to establish this. Fish have fibres nearly four times thicker than those of Birds, which have the smallest of all animals. Next to Fish come Insects, then Reptiles, then AIammalia. In each of these different classes, however, an extensive range of bulk is observ able, not only in the different genera, but in the same animal and the same muscle, some fibres being occasionally three, four, or more times the width of others. In general the fibres of the heart are smaller than those of other striped muscles. The varieties in the average bulk in different classes have a close connection with differences of nutrition and of their irritability, which will be reverted to.
3. Figure.—This is subject to some variety, depending on their number and manner of package. Sometimes, as in some parts of In
sects, they are flattened ; but when they are isolated, or loosely aggregated, they are more or less cylindrical. In all the cases, however, where many fibres are arranged side by side, as in the Vertebrata, the larger Insects, and Crustacea, they are irregularly polygonal, the contiguous sides being flattened, evidently from the effect of package. Yet some interspaces are always left for the passage of bloodvessels, nerves, and areolar tissue among and be tween them. Their form may be most readily displayed by a transverse section of a muscle that has been dried en masse, as long ago shown by Leeuwenboeck (fig. 286).
4. Colour.—The colour of muscle depends partly on the colour of its elementary fibres, partly on the blood contained in its vessels, and there is strong reason to believe that the colouring matter of both is the same, or nearly so. That the fibres have always a colour of their own is at once evident on inspection under the microscope. It is generally more or less of a reddish-brown, but varies much in different animals and in different muscles, and even in the same muscle according to its state of development and activity. In Reptiles and Fish generally, and in Crustacea, the flesh is white, sometimes pinkish, but in some fishes, as the Gumard, the gill-muscles are red. These varieties of colour are attended with none of struc ture. In Birds the colour varies much, being often white and deep red in the same animal, but generally the pectoral muscles are very dark.
The most deeply coloured muscle I have seen was the great pectoral muscle of the Teal ( Querqucdula (recce,), killed after migration. In Ilammalia the colour is ordinarily red, being deeper in the Carnivora 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 hamatosine 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.