270 ESSENTIALS OF ZOOLOGY 



the patella, or knee-cap, and the deep notch below is the 

 intercondyloid fossa. 



The patella is the large sesamoid which receives the extensor 

 tendons of the muscles in front of the femur, concentrating 

 their action on the tibia. The intervention of the patella 

 converts the lower part of the tendons into a strong ligament, 

 called the patellar ligament, and it is inserted into the tibia at 

 the tibial crest. Smaller sesamoids may also occur, as in the 

 rabbit, on the posterior aspect of the knee-joint. 



The tibia corresponds or is serially homologous to the 

 radius, and the fibula to the ulna. The tibia is the large shin 

 bone and in antero-medial position ; the fibula is postero- 

 lateral. The fibula is usually much smaller and, as in the 

 rabbit, fails to reach the knee-joint, and it is more or less fused 

 with the tibia. In the rabbit it articulates by its upper end 

 with the tibia, and below it is fused to it. 



The tarsal bones form the ankle-joint. They are as follows : 



Astragalus (tibiale-intermediale), Calcaneum (fibulare), 



Navicular (centrale) 

 Cuneiforms 

 First Second Third Cuboid 



I II III IV-Vtarsalia 



The first cuneiform is absent in the rabbit, but a rudiment 

 of it is fused to the second metacarpal. It is absent also in 

 many mammals in association with the absence of the first 

 digit. 



There are four metatarsals in such mammals, the first being 

 absent, and the phalanges are three each. When the first 

 digit is present it bears two phalanges. This is the large or 

 inner toe and corresponds serially with the thumb. 



The movements are facilitated by the provision of joints 

 between the bones. These may be, as has been observed, 

 merely cartilaginous connexions which permit of a limited 

 degree of movement. More perfect powers of movement are 

 permitted at many joints, as those of the limbs, and in such 

 cases a pad of cartilage is preserved at the opposing ends of the 

 bones, and between them a synovial cavity is developed with a 

 membrane which provides for the surfaces being kept moist. 



