XIII 



PHYLUM CHORDATA 



365 



Neither ischium nor pubis unites ventrally with its fellow to form 

 a symphysis. 



In the hind-limb the femur (Fig. 

 978, fe.) is a comparatively short bone. 

 Its proximal extremity bears a promi- 

 nent trochanter (tr.) and a rounded 

 head (M.), the axis of which is at right 

 angles to the shaft of the bone, so that 

 the femur, and indeed the w^hole 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.2), representing 

 the distal tarsals, and followed by three distinct metacarpals, 



FIG. 978. Columba 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.U, 

 phalanx of fourth digit ; ti. ts. 

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

 tarsus ; tr. trochanter. 



