MECHANISM OF THE INFERIOR EXTREMITIES. 243 



it is much more extensive anteriorly and externally than pos- 

 teriorly and internally. 



Rotation, owing to the length of the neck of the os femoris, 

 is extremely well marked, and is indicated by the trochanter 

 major moving backwards and forwards. The radius of the 

 circle thus described, is the distance between the centre of the 

 head of the os femoris and the bulging external part of the tro- 

 chanter major. The rotation outwards or backwards is more 

 fully and easily performed than the reverse, owing to the num- 

 ber and favourable position of the muscles causing it, many of 

 which are specially appropriated to its production, and some 

 others partially so. This movement is arrested by the neck of 

 the bone striking against the acetabulum behind, and by the 

 tension of the capsular ligament in front. Rotation, forwards, 

 having but few muscles to produce it, and they neither special- 

 ly devoted to it, nor acting very advantageously for the pur- 

 pose, is arrested by the neck of the bone striking against the 

 fore part of the acetabulum, by the tension, behind, of the cap- 

 sular ligament, and also, by that of the ligamenturn teres. 

 When the convexity and the neck of the os femoris look di- 

 rectly forwards, it is indicated by the great toe pointing in the 

 same direction. 



2. Of the Motions of the Leg. 



The movement of the leg upon the thigh is that of flexion, 

 of extension, and a very partial degree of rotation. 



In flexion, the head of the tibia slides backwards upon the 

 condyles of the os femoris, which are prolonged behind, for the 

 purpose of extending this motion. It is checked, when carried 

 to an extreme, by the posterior margin of the tibia striking 

 against the os femoris, and by the tension of the ligament of 

 the patella. In the mean time, the lateral, the crucial, and the 

 posterior ligaments are relaxed. The patella, always station- 

 ary, and at the same relative distance in regard to the head of 

 the tibia, slides downwards upon the trochlea of the os femo- 

 ris, and in the flexed position sinks between the condyles, so as 

 to come in contact with the ligamentum mucosum. 



