CHARACTERS OF AVES. 491 



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

 Cassowary, Bustards, 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 outer toe and 

 the one next to it. The toes are mechanically flexed during 

 the sleep of the Bird, in virtue of an arrangement by which, 

 whilst all the flexor tendons pass behind the heel, one of 

 them runs in front of the knee. As the muscles, therefore, are 

 relaxed during sleep, and the weight of the body tends to flex 

 the knee, the tendon of this flexor is thereby put on the 

 stretch, and the toes are again bent involuntarily. 



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, constituting 

 the bill. The form of the bill varies enormously in different 

 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 ; 

 but in some cases as in the Parrots it is soft and fleshy, and 

 then, doubtless, is to some extent connected with the sense 

 of taste. It is essentially composed of a prolongation of the 

 hyoid bone (the glosso-hyal), which is sheathed in horn, and is 

 variously serrated or fringed. 



Salivary glands are invariably present, but they are rarely of 

 large size, and they have often a very simple structure. 



In accordance with the structure of the neck, the gullet in 

 birds is usually of great length, and it is generally very dilatable. 

 In the carnivorous, or Raptorial, and in the granivorous birds, 

 the gullet (fig. 215, o), is dilated into a pouch, which is situated 

 at the lower part of the neck, just in front of the merry-thought. 

 This is what is known as the " crop " or " ingluvies " (c), and 

 it may be either a mere dilatation of the tube of the gullet, or 

 it may be a single or double pouch. The food is detained in 

 the crop for a longer or shorter time, according to its nature, 

 before it is subjected to the action of the proper digestive 

 organs. The oesophagus, after leaving the crop, shortly opens 



