THE TIBIA. .383 



its inclination varies much. In some cases it nearly or quite reaches the superior 

 articular surface. Laterally, this tuberosity is rough for the ligaments of the knee- 

 joint. The same may be said of the side of the inner tuberosity, which towards the 

 back has a broad horizontal groove running along it for the tendon of the semi- 

 membranosus. The tubercle^ of the tibia is a triangular prominence on the front of 

 the upper end. Its lower part is rough for the tendon of the extensor quadriceps, 

 and its upper smooth for a bursa between this tendon and the bone. The top of 

 the tubercle is about an inch below the top of the bone ; it is lost below in the ridge 

 of the front of the shaft. 



The shaft ^ has three borders and three surfaces. The anterior border, the cresP 

 begins at the outer side of the tubercle, curves as it descends, at first a little inward, 

 then a little the other way through the middle of the shaft, where it is very sharp, 

 and, finally, at the lower third, becoming much less prominent, it sweeps to the 

 front of the inner malleolus. The inner border, the least marked of the three, begins 

 under the inner tuberosity near the back and goes to the back of the inner malleolus. 

 It is most distinct in the middle. The outer border, or interosseous 7'idge* begins below 

 the facet for the head of the fibula, runs downward and somewhat backward past the 

 middle of the shaft, and then, inclining forward, divides some two or three inches 

 above the lower end into two lines enclosing a space on the outer side of the lower 

 end, to which the fibula is bound by ligaments. The anterior of these divisions is 

 the more evident continuation of the ridge. The internal surface is subcutaneous : 

 generally convex above and concave below ; the outer, bounded behind by the in- 

 terosseous ridge, is at first external, but in the lower third twists to the front. The 

 posterior, in its upper and lower parts, faces also somewhat outward. It is crossed 

 in the upper third by the oblique line,^ which, running downward and inward from 

 the back of the fibular facet to the inner border, marks ofF a triangular space above 

 it which is occupied by the popliteus muscle. A vertical line, generally very faint, 

 running down for some distance from the oblique line partially divides this sur- 

 face into an inner broader and an outer narrower part : the former for the flexor of 

 the toes, the latter for the tibialis posticus. The mitrient foramen, the largest 

 in the body, is on this surface at the junction of the first and second thirds external 

 to the oblique line ; it runs down into the bone. The shaft is triangular on section in 

 the upper and middle thirds, being narrower and sharper in front in the middle one. 

 In the lower third the section becomes quadrilateral as the shaft broadens and the 

 anterior border sinks and turns inward. 



The lower extremity is thickest transversely. The internal malleolus^ is a 

 thick projection downward and inward from the whole of the inner side, to form 

 one boundary of the ankle. Its lower end is thick, reaching farthest down in front, 

 with a depression at the back for the lateral ligament of the ankle. The surface 

 looking towards the joint is articular ; it slants a little away from the median line of 

 the bone. The outer side of the lower end of the shaft is slightly concave, with a 

 tubercle both before and behind. The articular cartilage of the lower end is pro- 

 longed some two or three millimetres onto this outer side. Both in front and behind, 

 but especially in front, the bone presents a swelling, separated by a depression from 

 the lower border, above which the capsule is inserted. On the posterior surface a 

 broad groove for the tendons of the tibialis posticus and the flexor longus digitorum 

 runs obliquely downward and inward onto and along the hind border of the mal- 

 leolus. A faint groove for the flexor longus hallucis is sometimes seen near the 

 outer end of the posterior surface. The lower side forms the top of the ankle- 

 joint and is wholly articular. It is broader before than behind, as the sides converge 

 towards the back. It is concave from before backward. There is a slight antero- 

 posterior elevation in the middle, fitting into a depression on the top of the as- 

 tragalus. 



Variations. — The transverse axes of the knee- and ankle-joints are rarely 

 parallel. The shaft of the tibia is so twisted as to make the foot point outward. 

 The angle between the two axes varies from o to 48°, but is usually between 5° and 

 20°. The backward inclination of the top of the tibia varies considerably. When 

 excessive, it seems to imply an aptitude for the squatting position, as among the 

 natives of India, but no inability to assume the upright position. A continuation 



^ Tuberositas tibiae. ' Corpus tibiae. ' Crista anterior. '* Crista interossea. ^ Linea poplitea. ' Malleolus medialis. 



