GENERAL STRUCTURE. 



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1 1 



this bone, above the astragalus, there is frequently (as in fig. 

 1 108, a) a bony bridge over the channel for the extensor tendons of 

 the foot. The tarso-metatarsus (fig. 1106, t??i) immediately suc- 

 ceeds the tibia ; and consists proximally of the distal portion 

 of the tarsus, with which the 



three middle metatarsals of the M&\ iLii 



typical foot are usually com- 

 pletely welded to form a rod- 

 like bone, generally terminat- 

 ing in three distal pulley-like 

 surfaces for the articulation of 

 the phalangeals. In the Pen- 

 guins, however, intervals exist 

 between the three component 

 bones of the shaft ; and in 

 the Ostrich only two metatar- 

 sals enter into the compound 

 element. Generally the me- 

 dian metatarsal at its distal 

 end is thrust somewhat in ad- 

 vance of the other two (fig. 

 1 108, b). The form of the 

 distal condyles of this bone 

 is very characteristic of differ- 

 ent groups of Birds. In cases 

 where the hallux, or first digit, 

 is developed it is only the dis- 

 tal portion of its metatarsus 



which is attached to the posterior aspect of the tarso-metatarsus. 

 No adult Bird has any trace of a fifth digit, and the number of 

 digits may vary from two (Ostrich) to four (Parrots). In four-toed 

 Birds the phalangeals generally number 2, 3, 4, 5, reckoning from 

 the first (hallux) to the fourth digit. 



This increase in an arithmetical ratio of the phalangeals of the toes, in 

 proceeding from the inner to the outer side of the foot, obtains in almost 

 all Birds, and enables us readily to detect which digit is suppressed, when 

 the normal four are not all present. Variations of different kinds exist, 

 however, in the number and disposition of the toes. In many Birds — 

 such as the Parrots — the outermost toe is turned backwards, so that there 

 are two toes in front and two behind ; whilst in the Trogons the inner toe 

 is turned back with the hallux, and the outermost one is turned forwards. 

 In others, again, the outer toe is normally directed forwards, but can be 

 turned backwards at the will of the animal. In the Swifts, on the other 

 hand, all four toes are present, but they are all turned forwards. In 

 many cases — especially amongst the Anserine birds — the hallux is wholly 

 wanting, or rudimentary. In the Emeu, Cassowary, Bustards, and other 

 genera, the hallux is invariably absent, and the foot is three-toed. In the 



v 



A 



Fig. 1 108. — Ibis melanocephala. The distal 

 portion of the right tibio-tarsus (a) and left tarso- 

 metatarsus (b). 



