CHARACTERS OF AVES. 433 



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. The most internal 

 of the three toes which are directed forwards, consists of three 

 phalanges; the next has four phalanges ; and the outermost or 

 " little " toe is made up of five phalanges (fig. 170, 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, ob- 

 tains 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. 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 Natatorial 

 birds the hallux is wholly wanting, or is rudimentary. In the 

 Emeu, Cassowary, Bustards, and other genera, the hallux is in- 

 variably 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 outer toe 

 and the one next to it. 



The digestive system of birds comprises the beak, tongue, 

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

 wanting in birds, and the jaws are encased in horn, constitut- 

 ing the bill. The form of the bill varies enormously in differ- 

 ent birds, and it is employed for holding and tearing the prey, 

 for prehensile purposes, for climbing, and in some birds as an 

 organ of touch. In these last-mentioned cases the bill is 

 more or less soft, and is supplied with filaments of the fifth 

 nerve. In many birds, too, in which the bill is not soft, the 

 base of the upper mandible is surrounded by a circle of naked 

 skin, constituting what is called the " cere," and this, no doubt, 

 serves also as a tactile organ. 



The tongue of birds can hardly be looked upon as an organ 

 of taste, since it is generally cased in horn like the mandibles. 

 It is, in fact, principally employed as an organ of prehension ; 



VOL. II. 2 E 



