XIII 



PHYLUM CHORD ATA 



393 



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



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



j.^ There is a small sesamoid bone {i.e., a 



bone developed in a tendon), the 



p((tcU(( (pat.), on the extensor side of 



the knee-joint. Articulating with the 



femur is a very long bone, the tihio- 



tarsus (ti.ts.) produced on the anterior 



face of its proximal end into a large 



cnemial prucess {m.pr.) for the insertion 



of the extensor muscle of the thigh. 



Its proximal articular surface is slightly 



hollowed for the condyle of the fenmr, 



its distal end is pulley-like, not concave 



/*' like the corresponding extremity of the 



li is — 



■mlh.1- 



/J 



ph.l^ 



V 



phi 



-ts.mtts 



Fir;. lOSS.-Columba livia. Part of left foot of an iin- 

 hatched euibryu (luagiiified). The cartilage i.s dotted. 

 mtl.?., sectmd, mtl. ■!, t\i\v<\, and mtl . d, U>nvi\\ meta- 

 tarsal; fj. tibia ; U.l, proximal tarsal cartilage ; tl.ri, 

 distal tarsal cartilage. (Frum I'arker's Ziiotomy.) 



tibia of other Am n iota. 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 fihida (fi..) is 

 very small, much shorter than the tibia, 

 and tapers to a point at its distal end. 

 Followincf the tibio-tarsus is an 

 elonoated bone, the tarso-metatarsus 

 (Fig. \0ol, 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 k-ft liill^l-IillilJ en. pr. 

 cnemial process ; j'f. femur ; _fi. 

 fibula ; hd. head uf femur ; nUtx. 1, 

 first metatarsal ; ;)((/. patella ; ph.l, 

 phalanges of first digit ; ph.U, 

 phalanges of fourth digit ; ti. ts. 

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

 tarsus ; ir. trochanter. 



