CHAP. VI.] BURS^E. SYNOVIAL SHEATHS. 127 



the extremities of the bones. From the ligaments it may be 

 readily detached, and traced to the edge of the cartilage ; to this it 

 is very intimately adherent for some little distance, beyond which 

 it cannot be followed where the cartilage has been exposed to pres- 

 sure during the motions of the joint. In the foetus it is continued 

 over the whole cartilage (p. 90). 



In some of the more complex joints the sy no vial membrane forms 

 folds, which project more or less into the articular cavities, and con- 

 tain fat, which Clopton Havers and other anatomists erroneously 

 imagined to perform a glandular office, and to secrete the synovia. 

 The knee affords some remarkable examples of these folds, in what 

 are known as the alar ligaments. 



A great number of blood-vessels are distributed in the areolar 

 tissue upon the attached surface of the synovial membrane. In 

 inflammation, the membrane acquires a red hue by the repletion of 

 these vessels, and in a minute injection also it becomes coloured. 

 Excepting in very rare cases, the vessels cannot be traced beyond 

 the edge of the cartilage, where they form a series of loops. (See 

 page 93). 



Burses. A very simple form of synovial membrane is employed 

 to facilitate the gliding of a tendon, or of the integument, over an 

 osseous projection. It consists of a bag, generally closed at every 

 point, connected by areolar tissue with the neighbouring parts, and 

 secreting into its interior fluid, which lubricates its free surface. 

 Sometimes, when one of these bursse exists in the neighbourhood of 

 a large joint, it communicates freely with the cavity of its synovial 

 membrane, as in the bursa behind the rectus femoris above the 

 knee-joint, and that near the hip-joint behind the tendon of the 

 psoas and iliacus muscles. These synovial sacs are found in great 

 numbers throughout the body: some are superficial, or subcutaneous, 

 such as that between the skin and the patella, or that over the 

 great trochanter of the femur, or that over the olecranon : the 

 deep-seated bursae, however, constitute the largest proportion of 

 them ; these are almost always connected with tendons, and inter- 

 posed between them and the bones over which they play. On 

 opening a bursa, we often find its cavity traversed by bands, which 

 are either congenital, and approaching to the cellular or areolar dis- 

 position, or, as seems not unlikely, of the nature of adhesions, and con- 

 sequently a morbid production tending to the obliteration of its cavity. 

 Synovial Sheaths. These are synovial bags prolonged into the 

 form of sheaths, and surrounding long tendons, such as those of 

 the flexor or extensor muscles of the fingers and toes, as they lie in 



