TENDONS. 317 



Tendons. — Muscles vary not only in general form and in the relations of the 

 constituent fibre-bundles to the intrinsic skeletal framework, but also in the mode 

 of attachment to the parts on which they act. In many instances the fibre-bun- 

 dles impinge, perpendicularly or obliquely, directly upon a bone or cartilage. 

 The tendinous tissue arising from the fibre-bundles of the muscle here is attached 

 to the periosteum or perichondrium or to the bone directly. A broad attachment 

 is thus offered the muscle. Instances of this mode of attachment may be seen in the 

 attachment of the intercostal muscles and of many of the muscles attached to the 

 shoulder and hip girdles. 



In the case of most thin, flat muscles the muscle is continued at one or both 

 extremities into thin, tendinous sheets called aponeuroses, composed of connective 

 tissue. Well-marked instances may be seen in the transverse muscle of the abdo- 

 men (fig. 390), and the trapezius and latissimus dorsi muscles of the back (fig. 355). 

 The extent of development of these aponeuroses is generally inversely proportional 

 to the development of the muscle — the more extensively developed the muscle is 

 in a given individual, the less extensive the aponeurotic sheet.* 



Most muscles are continued at one or both extremities into dense, tendinous 

 bands which may be comparatively short and thick, like the tendon of Achilles 

 (fig. 413), or very long and narrow, like the tendon of the palmaris longus (fig. 370). 

 In this latter case the tendon represents in part the remnants of musculature 

 more highly developed in the lower vertebrates. In most instances, however, 

 the tendons are structures specifically differentiated for definite functions and are 

 composed of bundles of parallel connective-tissue fibrils held together by an inter- 

 lacing fibrous-tissue framework. The tendons usually contain a relatively small 

 amount of elastic tissue. 



The tendons are attached to the skeleton early in embryonic development. 

 As the bones enlarge the tendons become in part incorporated in the substance of 

 the bone and ossified. 



In some tendons sesamoid bones are developed in the neighbourhood of joints 

 over which the tendons pass. Examples of these are the patella at the knee-joint 

 (fig. 412) and the sesamoid bones of the thumb and great toe. 



Where muscles or tendons closely envelope a joint, there is usually formed a 

 close union between the connective tissue of the capsule of the joint and that of the 

 muscle or the tendon. Special bands may develop in the direction of the pull 

 of the muscle (lig. popliteum obliquum). 



Where tendons run for some distance across or beneath a fascia, they are 

 usually either bound to the fascia by a special investment, as near the wrist and 

 knee (fig. 366 and fig. 414), or are fused with the fascia, as in the case of the ilio- 

 tibial band. Fibrous tracts in the fascia may indicate stress under muscle con- 

 traction (the lacertus fibrosus of the fascia of the forearm). 



Often in broad aponeurotic attachments of muscles there is formed in the ten- 

 don near the attachment a fibrous archway [arcus tendineus] for the passage of 

 blood-vessels, nerves, muscles, or tendons. The tendinous arch is either fastened 

 at both ends to the bone, or at one end it is connected with a joint capsule or other 

 membrane. The dorsal attachment of the diaphragm (fig. 391) and that of the 

 adductor magnus to the femur (fig. 409) offer good examples of tendon arches. 



In digastric and polygastric muscles the transverse tendons which separate 

 the bellies are often composed of narrow, incomplete bands of fibrous tissue. 

 Such a transverse band is called an inscriptio tendinea (see Rectus Abdominis 

 Muscle, fig. 388). 



Tendon sheaths. — The tendons are held in place by sheaths composed of dense 

 connective tissue. These sheaths vary in different regions. In the most com- 

 plete form they confine tendons in osseous grooves which they convert into osteo- 

 fibrous canals on the flexor surface of the digits. The sheath is here called a 

 vagina fibrosa tendinis. It is strengthened by tendinous bands (vaginal liga- 

 ments). In other regions special dense bands or ligaments, retinacula tendinum, 

 confine a series of tendons in place, as at the ankle (fig. 417), or fascia may be 

 modified for this purpose, as at the back of the wrist (fig. 366) . A tendinous loop, 



* The terms fascia and aponeurosis are often loosely and interchangeably used. It seems 

 best to make a distinction by restricting the term fascia to membranous sheets of investment, 

 and aponeurosis to broad tendons. The latter may, however, be inserted into and form a part 

 of the former. 



