XIII 



PHYLUM CHORDA.TA 



393 



fid 



sagittal plane of the trunk, and is not directed outwards as in 

 Reptiles. Its distal end is produced into pulley -like condylcs. 



There is a small sesamoid bone (i.e., a 

 bone developed in a tendon), the 

 patella (pat.), on the extensor side of 

 the knee-joint. Articulating with the 

 femur is a very long bone, the tibio- 

 tarsus (ti.ts.) produced on the anterior 

 face of its proximal end into a large 

 cnemial process (cn.pr.) for the insertion 

 - of the extensor muscle of the thigh. 

 Its proximal articular surface is slightly 

 hollowed for the condyle of the femur, 

 its distal end is pulley-like, not concave 

 like the corresponding extremity of the 



fiat 

 cn.pi 



ti.ls 



-/*' 



mtli 



ph.* 



FIG. 103S.-Columba livia. Part of left foot of an un- 

 hatched embryo (magnified). The cartilage is dotted. 

 mtl. %, second, mtl. >, third, and mtl. k, fourth meta- 

 tarsal ; li. tibia ; tl. 1, proximal tarsal cartilage ; tl. 2, 

 distal tarsal cartilage. (From Parker's Zootomy.) 



tibia of other Arnniota. The study of 

 development shows that the pulley- 

 like distal end of the bone (Fig. 1038, 

 tl.l) consists of the proximal tarsals 

 astragalus and calcaneum which at 

 an early period unite with the tibia 

 and give rise to the compound 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 

 (Fig. 1037, ts. mtts), presenting at its 

 proximal end a concave surface for the tibio-tarsus, and at its distal 

 end three distinct pulleys for the articulation of the three forwardly- 



Fio. 1037. Columba livia. Bones 

 of the left hind-limb, m. ///. 

 cnemial process ; fe. femur ; fi. 

 fibula ; hd. head of femur ; mtts. 1, 

 first metatarsal ; pat. patella ; ph.l, 

 phalanges of first digit ; ph.U, 

 phalanges of fourth digit ; ti. ts. 

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

 tarsus : tr. trochaiiter. 



