XIII 



PHYLUM CHORDATA 



393 



sagittal plane of the trunk 

 Reptiles. Its distal end is 



,?»• 



and is not dii'ected outwards as in 

 IS produced into pulley-like condyles. 

 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- 

 tarsViS (ti.ts.) produced on the anterior 

 face of its proximal end into a large 

 cnemial process (cn.pr.) for the insertion 

 of the extensoP 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 



fZ.' 





Fig. lOSS.-Columba livia. Part of left toot of an un- 

 hatched embryo (magnified). The cartilage is dotted. 

 mti. S, second, mi^S, third, and mtl. /t, iourt\i meta- 

 tarsal ; a. tibia ; tl. 1, proximal tarsal cartilage ; tl. S, 

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



tibia of other Amniota. 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.— Coliunba livia. Bones 

 of the left hind-limb, cn.pr. 

 cnemial process ; fe. femur ; Jl. 

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

 first metatarsal ; pat. patella ; ph.l, 

 phalanges uf first digit ; ph.U, 

 phalanges of fourth digit ; ti. ts. 

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

 tarsus ; tr. trochanter. 



