XIII 



PHYLUM CHORDATA 



365 



hd_ 



fiat 

 cn.pi 



ti.ts 



Neither ischium nor pubis unites ventrally with its fellow to form 

 a symphysis. 



In the hind-limb the femur (Fig. 

 978, /<?.) is a comparatively short bone. 

 Its proximal extremity bears a promi- 

 nent trochanter (tr.) and a rounded 

 head (hd.) y the axis of which is at right 

 angles to the shaft of the bone, so that 

 the femur, and indeed the whole limb, 

 lies in a plane parallel with the sagit- 

 tal plane of the trunk, and is not 

 directed outwards as in Reptiles. Its 

 distal end is produced into pulley- 

 like condyles. There is a small sesa- 

 moid bone, the patella (pat.), developed 

 on the extensor side of the knee-joint. 

 Articulating with the femur is a very 

 long bone, the tibio-tarsus (ti.ts.) pro- 

 duced on the anterior face of its proxi- 

 mal end into a large cnemial process 

 (cn.pr.) for the insertion of the ex- 

 tensor muscle of the thigh. Its 

 proximal articular surface is slightly 

 hollowed for the condyle of the femur, 

 its distal end is pulley-like, not con- 

 cave like the corresponding extremity 

 of the tibia of other Amniota. The 

 study of development shows that the 

 pulley-like distal end of the bone 

 (Fig. 979, tl.l) consists of the proximal 

 tarsals astragalus and calcaneum 

 which at an early period unite with 

 the tibia and give rise to the com- 

 pound shank-bone of the adult. The 

 fibula (fi.) is very small, much shorter 

 than the tibia, and tapers to a point 

 at its distal end. 



Following the tibio-tarsus is an 

 elongated bone, the tarso-metatarsus 

 (ts. mtts.), presenting at its proximal 

 end a concave surface for the tibio- 

 tarsus, and at its distal end three dis- 

 tinct pulleys for the articulation of 

 the three forwardly-directed toes. In 

 the young Bird the proximal end 



of this bone is a separate cartilage (Fig. 979, tl.), representing 

 the distal tarsals, and followed by three distinct metacarpals, 



ph.* 



FIG. 978. Coiumba livia. Bones 

 of the left hind-limb. en. pr. 

 cnemial process ; fe. femur ; ft. 

 fibula ; hd. head ; mtts. 1, first 

 metatarsal ; pat. patella ; ph. 1, 

 phalanx of first digit ; ph.lt, 

 phalanx of fourth digit ; ti. ts. 

 tibio-tarsus ; ts. mtts. tarso-meta- 

 tarsus ; tr. trochanter. 



