Class 6. Mammalia. 



479 



the acetabulum at the posterior end (whilst in Reptilia the ilium 

 is directed forwards or antero-ventrally) . The ischia and pubes of 

 each side fuse, and the pubes also anchylose in the mid line ; as 

 may also the ischia ; occasionally there is no connection between the 

 two halves of the pelvis {e.g., certain Insectivora) . In the adult all 

 three bones of each side are completely fused* 



In the Monotremes and the Marsupials, attached to the anterior edge of the 

 pubis,, is a pair of forwardly-directed bones, the so-called marsupial bones; 

 they may be regarded as ossifications in the tendons of the abdominal muscles. 



Hind limbs. The tibia is always stronger than the 

 fibula^ which is . often very thin, or indeed imperfect at its lower 

 end, where it is usually fused with the tibia. 

 A large patella lies over the knee-joint, 

 anteriorly. There are only two bones in the 

 proximal row of the tarsus; the astra- 

 galus within, and the cal c aneum, with 

 the much projecting heel, postero-externally. 

 Movement occurs between the lower end of 

 the fore-leg and the astragalus (or sometimes 

 the calcaneum), whilst movement between 

 the tarsals themselves is usually much limited 



(c/., the very difEerent conditions in Reptiles 



and Birds) . In the distal row there are four 



bones,t as in the handj between the two 



rows on the inner side there is a centrale 



{navicular e) . Metatarsus and toes as regards 



the number of phalanges, etc., agree with 



the metacarpus and fingers ; and with regard 



to special developments, such as reduction 



in the number of toes, the relations are 



usually similar. Sometimes, however, the 



hand is modified differently from the foot 



{e.g., in leaping or digging animals). 



Other sesamoid bones are found besides 



these already mentioned (pisiform, patella), namely ,_ 



below the joints between each metacarpal and the 



first phalanx (also between each metatarsal and the 



first phalanx of each toe), there are two small bones ; 



and below the joint, between the last and the pen- 

 ultimate phalanx of the finger and toe, one sesamoid bone ; other smaller ones 



may also occur, but less frequently. 



rig. 391. Tibia of a one 

 year old Horse to show the 

 epiphyses e and e'. — Orig. 



* The two halves may be anchylosed in the mid-ventral line, and the ilia may also 

 fuse with the sacral vertebree. In some forms [eg., certain Edentata), the ischia may 

 be fused with the posterior false sacral vertebrse. 



t Cuneiforms 1, 2, and 3, and the cuboid, the latter consisting of distal tarsals^ 

 4 and 5. 



