164 



OSTEOLOGY. 



Fig. 107. Right Patella. 

 Anterior Surface. 



Fig. 108. Bight Patella. 

 Posterior Surface. 



THE PATELLA. (Figs. 107 and 108.) 



The Patella is a small, flat, triangular bone, situated at the anterior part of the 

 knee-joint. It resembles the sesamoid bones, from being developed in the tendon 

 of the Quadriceps extensor, and in its structure, being composed throughout of 

 dense cancellous tissue ; but it is generally regarded as analogous to the olecranon 

 process of the ulna, which occasionally exists as a separate piece, connected to the 

 shaft of the bone by a continuation of the tendon of the Triceps muscle. 1 It 

 serves to protect the front of the joint, and increases the leverage of the Common 

 extensor by making it act at a greater angle. It presents an anterior and posterior 

 surface, three borders, a base, and an apex. 



The anterior surface is convex, perforated by small apertures, for the passage 

 of nutrient vessels, and marked by numerous rough 

 longitudinal striae. This surface is covered, in the 

 recent state, by an expansion from the tendon of the 

 Quadriceps extensor, separated from the integument 

 by a bursa, and gives attachment below to the liga- 

 mentum patellse. The posterior surface presents a 

 smooth, oval-shaped, articular surface, covered with 

 cartilage in the recent state, and divided into two facets 

 by a vertical ridge, which descends from the superior 

 towards the inferior angle of the bone. The ridge 

 corresponds to the groove on the trochlear surface of 

 the femur, and the two facets to the articular surfaces of 

 the two condyles ; the outer facet, for articulation with 

 the outer condyle, being the broader and deeper, serves 

 to indicate the leg to which the bone belongs. Below 

 the articular surface is a rough, convex, non-articular 

 depression, the lower half of which gives attachment to 

 the ligamentum patellas ; the upper half being separated 

 from the head of the tibia by adipose tissue. 



Its superior and lateral borders give attachment to 

 the tendon of the Quadriceps extensor; the superior 

 border, to that portion of the tendon which is derived 

 from the Eectus and Crureus muscles; and the lateral 

 borders, to the portion derived from the external and 

 internal Vasti muscles. 



The base or superior border is thick, directed upwards, and cut obliquely at 

 the expense of its outer surface ; it receives the attachment, as already mentioned, 

 of part of the Quadriceps extensor tendon. 



The apex is pointed, and gives attachment to the ligamentum patellse. 

 Structure. It consists of dense cancellous tissue, covered by a thin compact 

 lamina. 



Development. By a single centre, which makes its appearance, according to 

 Beclard, about the third year. In two instances, 1 have seen this bone cartilaginous 

 throughout, at a much later period (six years). More rarely, the bone is developed 

 by two centres, placed side by side. 



Articulations. With the two condyles of the femur. 



Attachment of Muscles. The Eectus, Crureus, Yastus internus, and Vastus 

 externus. These muscles, joined at their insertion, constitute the Quadriceps ex- 

 tensor cruris. 



1 Professor Owen states, that, "in certain bats, there is a development of a sesamoid 

 bone in the biceps bracbii, which is the true homotype of the patella in the leg," regarding the 

 olecranon to be homologous, not with the patella, but with an extension of the upper end of 

 the fibula above the knee-joint, which is met with in some animals. (" On the Nature of Limbs," 

 r>p. 19, 1:4.) 



