Each fiber is elongated, cylindrical, and multi-nucleated. Howell states its length does not exceed 36 mm. but the species is not stated. The average length varies in different muscles in the same animal and in the same muscles in different kinds of animals. Hammond says that the average length of goat fibers is said to be greater than those of beef, and those of beef greater than fibers in mutton. The average diameter also varies in different muscles, in different species, and in the same animal with age. Hammond found the average diameter of a particular muscle of Suffolk ram at birth, at 5 months, and at 4 years to be 12.8u, 40.3u, and 54.1u, respectively. It is commonly accepted that the number of fibers does not increase after birth so that, in spite of increase in diameter with age, the size of the muscle is largely determined by the number and length of the fibers rather than their thickness.

Each fiber is enclosed in the sarcolemma, a thin, elastic membrane. A thin, delicate band occurring regularly along the length of the fiber divides the fiber into its structural units, the sarcomeres, a fiber being composed of a succession of sarcomeres. A dark band shows in the middle of each sarcomere, a lighter band on either side. The material between the fibrils is known as the sarcoplasm and the relative amount of sarcoplasm varies in muscles of different animals.

The contracting tissue, the fibrils, run the length of the fiber. The fibrils consist of alternating light and dark segments which form the cross striations of the fibers. See Figs. 21 and 22. The cross bands are supposed to be of denser structure than the material between these bands. Fig. 22 shows the immense number of these cross striations. The cross striations also show plainly in the fibers of Fig. 21.

All the material within the sarcolemma is known as the muscle plasma and in the living muscle is in a semi-liquid state.

Effect of exercise and maturity upon fibers. Beard in his study of "Tough and Tender Meat" states that "muscle fibers attain their maturity by use and age. Development implies increase in size rather than maturity." Beard made a microscopic study of different muscles of five different animals. The animals selected for study were a heifer, a short-fed steer, a prime steer, an old cow, and a veal calf. Some of his observations and conclusions are as follows:

"Connective tissue seems more apparent in thinner fleshed beef.

"There was intracellular fat (fatty degeneration) in the fat steer.

"The fat steer had more intrafascicular, intercellular fat than any animals studied.

"The inherent properties of the endomysium contribute to the toughness of meat more than does the size of the fiber.

"The muscle fibers from the region of the toughest meat contain the densest sarcoplasm, while fibers from the tenderest muscles contain the lightest sarcoplasm.

"Intramuscular and particularly intrafascicular fat lessens the toughness of meat.

The Muscle Fibers 47

Fig. 22. - Longitudinal section of muscle fibers of beef, showing cross striations. Magnification approximately x 1400.

"Where only a small amount of intramuscular fat is found it is almost always interfascicular. As the interfascicular fat increases in amount intrafascicular fat appears which is first intercellular and in case of excessive adiposity intracellular also."

Beard states that "while those muscles which are brought most into activity and those performing the greatest amount of work may not possess a larger fiber than those muscles which are less active, the more active muscles do have a thicker perimysium and apparently more substance in the fiber, as manifested in the staining characters of the sections."

Toughness of meat. Since meat is composed of muscle fibers and connective tissue, toughness of meat must be due to either of these, or to both. Toughness of connective tissue depends upon the proportion of elastin and collagen; upon the thickness or density of the tissue, which is brought about by use and activity; and possibly upon age, for it is common knowledge that meat from old animals and fowls, whether domestic or wild, is tougher than that from young animals. Toughness of the muscle fiber depends upon the development and the density of the fiber due to activity, and possibly to changes brought about by age.

The deposition of fat, either intramuscularly, intrafascicularly, or intracellulary), tends to lessen the toughness. This is shown by the results of mechanical tests with the dynamometer, which are given in Table 27. The muscle used for these tests was the "eye" of beef or the longissimus dorsi. The judges, as shown by scores, agreed with the mechanical tests, the roasts from the fattened animals being judged more tender than the roasts from the lean animals. The possibility of toughness due to power to bind water, which may be influenced by the reaction and salt content of the fibers, thereby affecting the turgidity of the fibers, and the possibility of toughening the fibers by heat coagulation are still to be considered.