80 ANATOMY OF VERTEBRATES. 



consists of the foot-bones of three digits coalesced, and often of a 

 fourth ligamentously joined thereto. This always small and 

 short seeming appendage is the distal end of the metatarsal of the 

 * hallux ' or innermost digit of the pentadactyle foot. The three 

 coalesced bones are the metatarsals of the second (ii), third (iii), and 

 fourth (iv) toes, fig. 34. In their original position the proximal end 

 of the third metatarsal is behind those of the second and fourth 

 which meet in front of it. A fossa below the meeting shows, 

 afterwards, two fore-and-aft canals which diverge to outlets at 

 the back part indicative of the breadth there of the middle meta- 

 tarsal. When, as in Aquila, there are two foramina in front of, 

 as well as two behind, the upper part of the metatarse, the inter- 

 space of the former shows the extent to which the mid-metatarsal 

 intervened between the others anteriorly, and this structure is 

 concomitant with a great excess of the transverse over the fore- 

 and-aft diameter of the proximal end of the metatarse. In most 

 birds a fore-and-aft canal also remains to indicate the primitive 

 distinction of the outer (iv) from the middle (iii) metatarsal near 

 their distal ends. 



The metatarse, fig. 34, 68, presents a proximal end with two 

 articular cavities (^ ento- ' and ' ecto-condylar ') and the intercon- 

 dylar space, a shaft with its processes, grooves, and perforations, 

 and a distal end divided (save in Struthio) into three trochlear con- 

 dyles for the three principal (ii, iii, iv) toes : in most birds, also, 

 there is a rough depression on the distal half of the inner meta- 

 tarsal, for that end of the innermost or first (i) metatarsal. The 

 proximal end varies in the proportions of its transverse and antero- 

 posterior diameters, in the depth of its articular surfaces, and 

 configuration of the intercondylar surface. As a rule the ento- 

 condylar surface is largest and deepest ; the ectocondylar surface 

 is nearly flat in the Eagle. A tuberosity rises from the fore part 

 of the intercondylar space in birds which sleep standing on one 

 leg (Gr«Z/<s and some others): it passes into the corresponding 

 space of the tibia, the bar anterior to which affords so much 

 resistance to flexion of the leg as counteracts the effect of oscilla- 

 tions of the body : it requires a muscular effort to bring the 

 tuberosity over that bar, and the elastic lateral ligaments are 

 then put on the stretch ; but as soon as the bar is passed the 

 tuberosity slips into the depression above with a snap or jerk. 

 One or more longitudinal ridges at the back of the upper end 

 of the metatarsal are called ' calcaneal ; ' they intercept or bound 

 tendinal grooves which, in some instances, are bridged over by 

 bone and converted into canals : the ridges may be expanded and 



