VERTEBRATA: AVES. 583 



The tarso-metatarsus is followed inferiorly by the digits of 

 the foot. In most birds the foot consists of three toes directed 

 forwards and one backwards four toes in all. In no wild 

 bird are there more than four toes, but often there are only 

 three, and in the Ostrich the number is reduced to two. In 

 all birds which have three anterior and one posterior toe, it is 

 the posterior thumb or hallux (that is to say, the innermost 

 digit of the hind-limb) which is directed backwards ; and it 

 invariably consists of two phalanges only, its metatarsal being 

 incomplete and united as a rule to the tarso-metatarsus by 

 ligament only. The most internal of the three anterior toes 

 (the "index") consists of three phalanges; the next ("middle") 

 has four phalanges ; and the outermost toe (" annularis ") is 

 made up of five phalanges (fig. 325, A). This increase in an 

 arithmetical ratio of the phalanges 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 sup- 

 pressed, when the normal four are not all present. Variations 

 of different kinds exist, however, in the number and disposi- 

 tion 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 toe is turned 

 forwards. In others, again, the outer toe is normally directed 

 forwards, but can be turned backwards at the will of the ani- 

 mal. In the Swifts, on the other hand, all four toes are pres- 

 ent, but they are all turned forwards. In many cases 

 especially amongst the Natatorial birds the hallux is wholly 

 wanting, or is rudimentary. In the Emeu, Cassowary, Bus- 

 tards, and other genera, the hallux is invariably absent, and 

 the foot is three-toed. In the Ostrich both the hallux and the 

 next toe (" index ") are wanting, and the foot consists simply 

 of two toes, these being the third and fourth digits. The toes 

 are mechanically flexed during the sleep of most birds by the 

 action of a special muscle which runs from the pubis outside 

 the knee to join one of the flexors of the toes (the flexor digi- 

 torum perforatus), and which is therefore put on the stretch 

 whenever the leg is bent upon the thigh. 



The digestive system of birds comprises the beak, tongue, 

 gullet, stomach, intestines, and cloaca. Teeth are invariably 

 wanting in living birds, and the jaws are encased in horn, con- 

 stituting the bill. Dental papillae, sometimes covered with a 

 cap of dentine, have, however, been observed in the embryos 

 of some Parrots. In the extinct Odontopteryx, moreover, the 

 osseous substance of the jaws is prolonged into tooth -like 



