LEG OF BIRDS. 27 



POSTERIOR EXTREMITY, OR LEG, OF BIRDS. 



The hind limb consists of three principal divisions : — 



I. The thigh, or femur. II. The leg, or cms. III. The foot, or jpes. The 

 latter has a more or less undivided portion, called, in ornithological language, the 

 tarsus, but the skeleton of which corresponds to the united three middle meta- 

 tarsal bones of mammals, to which is also joined the lower or distal part of the 

 tarsus proper. The other portion of the true tarsus is united to the lower end of 

 the tibia. The ankle-joint is therefore not situated between the tibia and tarsus, 

 as in mammals, but in the middle of the tarsus itself. 



To the lower end of the tarso-metatarsal bone the three principal (second, 

 third, and fourth) digits, or toes, are attached. The first digit has a separate, or 

 accessory, metatarsal bone, of which only the lower end is developed, and which 

 is usually attached loosely to the hinder surface of the principal bone. 



The front, or dorsal, surface of the tarsus is called the acrotarsium, the back 

 the planta, as it corresponds to the sole of the foot of man and plantigrade 

 animals (Nat. Hist. Museum). 



Spurs on leg. 



Spurs, consisting of a conical bony core covered by a horny sheath, are de- 

 veloped on the posterior or inner surface of the metatarsus in some birds. There 

 is usually but one, though occasionally two, on each foot. There is one spur on 

 the Grey Jungle-Fowl (G. sonnerati) and two spurs in the Red Spur-Fowl 

 {G. spadicea). Spurs are usually present in the male sex only. 



Covering of the feet. 



In some birds, notably the Owls and Grouse, the tarsi, and even the toes, are 

 more or less covered with feathers or bristles. Usually that part of the limb 

 which is devoid of feathers is covered, like the bill, by a hardened, thickened, 

 modified integument, varying in texture from horny to leathery. This sheath is 

 called the podotheca. It is more corneous or horny on land birds, and softer and 

 Inore leathery in those that habitually live on the water. Its surface may be — 



(i.) Scutellated, disposed in scales or scutella (like a Pheasant's), 

 (ii.) Reticulated, cut up by cross lines, leaving polygonal plates between them 

 (as in a Macaw). 



(iii.) Granulated, or rugose, covered with little tubercles. 



(iv.) Cancellated, covered with a lattice-work of cross lines not strong enough 

 to produce distinct plates (hke the web of a "Wild Duck's foot). 



When the planta (sides and back of the tarsus) is covered with one pair of 

 plates or laminae, one on each side, meeting behind in a sharp ridge, the condition 

 is called laminiplantar. 



When there is no division of the podotheca in front (along the acrotarsium), 

 or only two or three scales close to the toes, it is said to be " booted," " greaved," 

 or holothecal. This occurs in the Thrushes and many other Passerine birds but 

 even these when young show scutella, which disappear by progressive fusion with 

 age (Nat. Hist. Museum). 



