400 HUMAN ANATOMY. 



much broader transversely than vertically. The outer three-fifths or so, which 

 plays on the external condyle, is concave transversely and the inner two-fifths con- 

 vex. The convexity begins with a vertical prominence which marks the greatest 

 thickness of the bone and appears to divide the hind surface into two parts, as a 

 horizontal section shows, the surface receding from it on either side. Neverthe- 

 less, the whole inner part is convex, as described. Vertically, both sides are slightly 

 concave. A close examination of a fresh specimen shows, what rarely is to be seen 

 on the dry bone, that the articular surface is to be further subdivided. A narrow 

 vertical facet is seen along the inner side, constituting a surface which rests on the 

 edge of the inner condyle in extreme flexion. The rest of the articular surface is 

 divided into three horizontal zones, one above another, by two transverse lines. The 

 top of the bone is very thick, most of it being occupied by the insertion of the 

 rectus. The capsule of the knee-joint is inserted all around the articular surface 

 some two or three millimetres from its edge, so that a little of the border is enclosed 

 in the joint. Several nutrient foramina are found on the anterior surface. 



Development. — The patella appears as a cartilaginous point in the course of 

 the third foetal month. Ossification begins by the deposit of several granules some 

 time between two and five years. These soon unite into a central mass, from which 

 ossification spreads, more rapidly, however, in the deeper parts. The bone is not 

 fully formed till after puberty, perhaps not before eighteen. 



THE LIGAMENTUM PATELLA. 



This name is applied to the tendon of the quadriceps extensor muscle, in which 

 the patella is a sesamoid bone (Fig. 416). It is a strong, flattened, fibrous band 

 some two inches long. Just below the knee-pan it is at least one and one-quarter 

 inches broad, but at its insertion into the front of the upper part of the tuberosity 

 of the tibia its breadth is not over one inch. The line of insertion is oblique, the 

 outer end being the lower. Just above the insertion a synovial bursa lies between 

 the tendon and the bone. A mass of fat above the bursa separates the tendon from 

 the capsule. The tendon is fused at the sides with fibrous expansions from the 

 quadriceps. » 



THE KNEE-JOINT. 



This is a compound joint between the femur and the tibia, the patella being a 

 sesamoid bone in the tendon of the extensor of the leg, incorporated in the front of 

 the capsule. The patella is in relation to the femur only, and sometimes it is con- 

 venient to consider the knee-joint as the sum of three distinct ones, — namely, that 

 between femur and patella, and one for each condyle with the tibia. The joint is 

 enclosed by a capsule partially subdivided in many ways. Fibro-cartilaginous disks, 

 the semilunar cartilages on the top of the tibia, tend to subdivide the joint below 

 each condyle into an upper and a lower half. The crucial ligaments nearly cut off 

 communication between the parts of the joint under each condyle. The mucous 

 ligament assists in this, and with the alar ligaments tends to isolate the patella. 



Discussion of the knee-joint calls for the description of the following component 

 structures : 



The Capsule and its Accessories. 



The Semilunar Cartilages and their Accessories. 



The Crucial Ligaments. 



The Subpatellar Fat with the Ligamentum Mucosum and the Ligamenta 

 Alaria. 



The Synovial Membrane. 



Certain Bursae. 



The capsule (Fig. 416) arises from the femur, mingling with the periosteum, 

 a little above the anterior articular surface ; from the sides of the condyles as high 

 as the level of the lateral tuberosities ; from the back one centimetre beyond the 

 highest point that the cartilage reaches on the top of the condyles ; and from a 

 slightly lower level above the intercondyloid notch. It is attached in front around 

 the articular surface of the knee-pan and inferiorly to the tibia all around, but a 



