STRUCTURE AND FUNCTION OF MAMMALIAN TENDON.

Summary 1. Tendons are a specialized form of connective tissue uniting muscle and bone and as such have functions essential to normal mobility. 2. The structural unit of tendon is the fibre, up to 300 µ in diameter, consisting of fibrils of collagen, encircled by the anastomosing processes of fibroblasts. Fibres are arranged in fasciculi but may pass from one to another so that successive cross-sections show slightly different appearances. Such interweaving ensures the equal distribution of muscular tension over the whole area of insertion, particularly where the movement of a joint alters the angle between tendon and bone. 3. There are two hypotheses concerning the nature of the muscle-tendon junction. First, the tension developed within a muscle fibre might be transmitted to the helical perisarcolemmal fibres of connective tissue surrounding its length and thence to the tendon, and secondly, and more probably, the tension might be transmitted directly from the end of the myofibrils to the interdigitating fibrils of the tendon. The interdependence of muscle and tendon is further illustrated by their longitudinal growth since the changing length of the muscle belly relative to the distance between the bony attachments determines the rate of growth and relative length of tendon. 4. The orientation of fibrous tissue in the tendon supports the hypothesis of a mechanical influence upon tendon growth, but there is no strict relationship between muscle strength and tendon thickness and various muscles differ in the ratio of their total fascicular cross-sectional area to tendon thickness. It would appear that this difference develops post-natally and, since a red postural muscle has a relatively thick tendon, it has been suggested that the duration as well as the level of transmitted tension might influence the growth of collagen. 5. Tendon thickness may increase proportionately with muscle cross-sectional area in conditions which cause the muscle to hypertrophy, but when the muscle is de-nervated or excised in the young animal growth of tendon thickness occurs in such a manner as to suggest that the growth of collagen is determined by the history of the total tension transmitted. 6. The tendon has a tensile strength which is probably four times as great as the maximum tension that it has to transmit in vivo and an even greater margin of safety is present in penniform muscles which transmit less maximum isometric tetanic tension per unit fascicular cross-sectional area than do fusiform muscles. Although the wave form seen on the surface of a tendon when at rest is eliminated by less than 10 % of the maximum tension which its muscle is liable to transmit, it is possible that the normal range of tensions transmitted in vivo might fall within that part of the stress-strain curve where the tendon is still easily extensible.