288 



AVES. 



of the latter disposition the power of the back 

 stroke of the foot is increased. 



The Tarsus can only be recognized as a 

 distinct segment of the leg when the bones 

 of a very young Bird are examined. But 

 in the Ostrich, even when it has attained 

 a third of its natural size, the Astragalus re- 

 mains ununited to the metatarsus. It is a 

 flattened transversely oval bone, convex in the 

 middle of its upper surface, and irregularly 

 flattened below, where it is adapted to the 

 three still partially separated bones of the 

 metatarsus. A rudiment of the os calcis may 

 be observed in the detached bone which is 

 found in the tendons of the extensors of the 

 foot near their insertion. The Capercailzie 

 ( Tetrao urogallus) affords a good example of 

 this structure. The process (m, Jig. 131) in 

 which the above tendons are inserted, and 

 which is very prominent in the Hasores, Gral- 

 latores, and Natatores, must also be regarded 

 as appertaining to the tarsal series, since it com- 

 mences by a separate ossification. 



In most birds, however, the tendo Achillis 

 has no sesamoid bone to add to its leverage, and 

 in all birds the astragalus is soon anchylosed to 

 the metatarsus, constituting with it one elongated 

 tarso-metatarsal bone (A, fg. 125, n, Jig. 131). 

 Traces of the number of laterally anchylosed 

 pieces of which the metatarsus is composed 

 are always more or less indicated by longitu- 

 dinal grooves. In the Penguins, indeed, the 

 anchylosis of the three metatarsal bones takes 

 place at their extremities only, and they are 

 consequently separated from each other in the 

 greater part of their extent. They are also 

 disproportionately short, and bent forwards 

 upon the tibia, so as to increase the surface of 

 support required by these birds when standing 

 in their usually erect position. In the Gralla- 

 tores and Struthiones, on the contrary, the 

 tarso-metatarsal bone is remarkably elongated, 

 the extraordinary length of leg in these birds 

 depending chiefly upon the extent of this seg- 

 ment of the limb. 



In the Stork and congeneric birds, which 

 sleep^on one leg, the ankle-joint presents a 

 mechanism analogous to that which we have 

 above described in the knee-joint. Here, how- 

 ever, the projection which causes the extension 

 of the elastic ligaments in the motion of the 

 joint is in the inferior bone. Dr. Macartney 

 thus describes the mechanism : " There arises, 

 from the fore-part of the head of the metatarsal 

 bone, a round eminence, which passes up be- 

 tween the projections of the pulley on the an- 

 terior part of the end of the tibia. This emi- 

 nence affords a sufficient degree of resistance 

 to the flexion of the leg to counteract the effect 

 of the oscillations of the body, and would 

 prove an insurmountable obstruction to the 

 motion of the joint, if there were not a socket 

 within the upper part of the pulley of the 

 tibia to receive it when the leg is in a bent 

 position. The lower edge of the socket is 

 prominent and sharp, and presents a sort of 

 barrier to the admission of the eminence that 

 requires a voluntary muscular exertion of the 



bird to overcome, which being accomplished 

 it slips in with some force like the end of a 

 dislocated bone."* It must be added, that the 

 elastic lateral ligaments contribute also to jerk 

 the metatarsal tubercle into the tibial cavities, 

 and to resist its displacement. 



The lower extremity of the metatarsus is 

 divided into three articular eminences, corres- 

 ponding to the ordinary number of anterior 

 toes. These eminences are convex from before 

 backwards, and the middle one, which is the 

 longest, is converted into a pulley by a mesial 

 groove which traverses it in the same direction. 

 The lateral surfaces are simply convex, and 

 very narrow ; of these the internal is the short- 

 est, except in the raptorial birds. At the extre- 

 mities of the grooves which indicate the lateral 

 juxtaposition of the metatarsal pieces, there are 

 ordinarily foramina extending from before back- 

 wards through the bone. 



A fourth articular surface is observable in 

 most birds on the inner and posterior side of 

 the metatarsal bone; this is situated on an ac- 

 cessory piece which always commences by a 

 separate ossification, although in some birds it 

 afterwards becomes anchylosed with the inner- 

 most of the other juxtaposed components of 

 the metatarsus. When this does not take place, 

 the metatarsus presents a rough, more or less 

 irregular, oval surface, for the firm ligamentous 

 attachment of the accessory bone which sup- 

 ports the back toe, usually termed the hallux or 

 posterior thumb. This articulating surface is 

 important as affording a good distinctive cha- 

 racter for identifying the bones of birds in a 

 fossil state, and the more so as its position is 

 indicative of the powers of grasping or perching 

 being placed low down, on a level with the 

 anterior toes, in those birds which enjoy the 

 insessorial power in the greatest perfection, and 

 being gradually removed higher and higher in 

 the Waders, until it is at length wholly lost, as 

 in the genus Cursorius, the Bustards, and the 

 Struthious family. In the Petrel, however, 

 this accessory metatarsal bone is wanting, al- 

 though the hallux is present, the two bones of 

 which are therefore united to the principal me- 

 tatarsal bone by long ligaments. The tarso- 

 metatarsal bone is further characterized by 

 sharp longitudinal ridges of bone on the pos- 

 terior surface, which afford attachment to the 

 aponeurotic thecae confining the tendons which 

 glide along the metatarsus to the toes. 



In birds, as in mammalia, the number of 

 toes is subject to great variety; if the spur of 

 the Gallinaceous tribe be regarded as one, we 

 may then reckon the ordinary number of five 

 in these birds, while in the Ostrich the toes are 

 reduced to two. Birds are, however, the only 

 class of animals in which the toes, whatever be 

 their number or relative size, always differ in 

 the number of their phalanges, yet at the same 

 time preserve a constancy in that variation. 



The following is a tabular view of the nume- 

 rical relation in the osseous parts of the feet of 



* See Transactions of the Royal Irish Academy, 

 vol. xiii. p. 20. 



