THE TIBIA, OR SHIN BONE 231 



facets by a vertical ridge, which descends from the superior border toward the 

 inferior angle of the bone. The ridge corresponds to the groove on the trochlear 

 surface of the femur, and the two facets tc the articular surfaces of the two con- 

 dyles; the outer facet, for articulation with the outer condyle, being broader and 

 deeper. This character serves to indicate the side to which the bone belongs. 

 Below the articular surface is a rough, convex, nonarticular depression, the lower 

 half of which gives attachment to the ligamentum patellae, the upper half being 

 separated from the head of the tibia by adipose tissue. 



Borders. The superior border (basis patellae") is thick, and sloped from behind, 

 downward and forward; it gives attachment to that portion of the Quadriceps 

 extensor which is derived from the Rectus femoris and Crureus muscles. 



The lateral borders are thinner, converging below. They give attachment 

 to that portion of the Quadriceps extensor derived from the external and internal 

 Vasti muscles. 



The apex (apex patellae) is pointed, and gives attachment to the ligamentum 

 patellae. 



Structure. It consists of a nearly uniform, dense caneellous tissue covered by a thin com- 

 pact lamina. The cancelli immediately beneath the anterior surface are arranged parallel with 

 it. In the rest of the bone they radiate from the posterior articular surface toward the other 

 parts of the bone. 



Development. From a single centre, which makes its appearance in the second or third, but 

 may not appear until the sixth year. More rarely, the bone is developed by two centres, placed 

 side bv side. Ossification is completed about the age of puberty. 



Articulations. With the two condyles of the femur. 



Attachment of Muscles. To four the Rectus, Crureus, Vastus internus, and Vastus 

 externus. These muscles, joined at their insertion, constitute the Quadriceps extensor cruris. 



Surface Form. The external surface of the patella can be seen and felt in front of the knee. 

 In the extended position of the limb the internal border is a little more prominent than the 

 outer, and if the Quadriceps extensor is relaxed the bone can be moved from side to side and 

 appears to be loosely fixed. If the joint is flexed, the patella recedes into the hollow between the 

 condyles of the femur and the upper end of the tibia, and becomes firmly fixed against the femur. 



Applied Anatomy. The main surgical interest about the patella is in connection with frac- 

 tures, which are of common occurrence. They may be produced by muscular action; that is 

 to say, by violent contraction of the Quadriceps extensor while the limb is in a position of semi- 

 flexion, so that the bone is snapped across the condyles; or by direct violence, such as falls on 

 the knee. Most fractures are due to muscular action; in fact, the patella is more often broken 

 by muscular action than is any other bone. In fractures by muscular action the line of fracture 

 is transverse. In fractures by direct force the line of fracture may be oblique, longitudinal, 

 stellate, or the bone variously comminuted. The principal interest in these cases attaches to 

 their treatment. Owing to the wide separation of the fragments, and the difficulty there is in 

 maintaining them in apposition, union takes place by fibrous tissue, and this may subsequently 

 stretch, producing wide separation of the fragments and permanent lameness. Various plans, 

 including opening the joint and suturing the fragments, have been advocated for overcoming 

 this difficulty. In many cases a portion of fascia or capsule gets between the fragments. In 

 such a condition operation is necessary. 



In the larger number of cases of fracture of the patella the knee-joint is involved, the car- 

 tilage which covers its posterior surface being torn, the synovial membrane lacerated, the lateral 

 fibrous expansions ruptured, and the patellar bursa torn open. In cases of fracture from direct 

 violence, however, this need not necessarily happen, the lesion may involve only the superficial 

 part of the bone; and, as Morris has pointed out, it is an anatomical possibility, in complete 

 fracture, if the lesion involve only the lower and nonarticular part of the bone, for it to take 

 place without injury to the synovial membrane. 



The Tibia, or Shin Bone (Figs. 185, 186). 



The tibia is situated at the front and inner side of the leg, and, excepting the 

 femur, is the longest and largest bone in the skeleton. It is prismoid in form, 

 expanded above, where it enters into the knee-joint, more slightly enlarged below. 



