PELVIC LIMBS 703 



The superior border is a narrow convex strip, sharply defined from 

 the posterior surface, but almost continuous with the prominent upper 

 part of the anterior surface. At each end it forms rounded acute 

 angles with the lateral borders. 



The lateral borders are wider than the superior border, and are 

 best defined as forming the lateral margins of the posterior surface. 



The posterior surface has the same outline as the anterior surface. 

 Except a small area at the tip and along the lower part of the lateral 

 margins, the entire surface is occupied by an articular facet which 

 glides upon the trochlea of the femur. To conform to this groove, the 

 surface is concave from above downward and convex from side to side. 

 The highest transverse convexity is along a longitudinal line which 

 lies nearer the outer than the inner border and divides the surface 

 into an outer smaller and an inner larger, flatter portion. The pos- 

 terior surface is covered by a layer of cartilage and gives attachment 

 around its borders to the synovial membrane of the joint. 



Nomenclature. The word patella was introduced into anatomy 

 by Celsus. It was used by the Romans for a flat pan, and is derived 

 from pdtere, to stand or lie open. The same idea is found in our 

 common word knee-pan. A synonym is rotula, a small wheel, which 

 was employed in the Middle Ages either because of the movements of 

 the patella in front of the joint, or because it was round like a wheel. 

 It is perpetuated in the French la rotule. The German has die 

 Kniescheibe, a translation of an old term, discus genu. 



Articulation. The patella is attached by ligament to the femur 

 and tibia, and moves in the trochlea of the femur. 



Ossification. The patella develops from a single centre of ossifi- 

 cation. 



HUMAN PATELLA. 



The transverse diameter of the human patella (Fig. 540) is slightly 

 greater than the vertical diameter ; hence the bone is relatively broader 

 and shorter than the corresponding bone of the cat. It is triangular 

 in outline ; the apex of the triangle is inferior, the base is superior. 

 It presents anterior and posterior surfaces and superior, external, and 

 internal borders. 



The superior border is very wide, and might be said to be the 

 upper part of the anterior surface, from which it is separated by a 

 transverse line. It is sharply defined by an arcuate margin from the 



