AVES. 



297 



ternal condyle of the femur; when it has 

 readied the region of the calcaneum, it passes 

 backwards through a synovial capsule, and 

 is inserted into the proximal phalanx of the 

 thumb, where it is perforated by the tendon 

 of the perforans muscle. 



The Flexor prqfundus perforans (52) arises 

 as two distinct muscles, the one from the back 

 of the femur and the other from the back 

 of the tibia and fibula; the tendons of these 

 two portions unite behind the metatarsal bone, 

 and send off tendons to the last phalanges of 

 the toes, which perforate those of the flexor 

 sublimis. 



The Extensor- longus communis digitorum 

 arises above from the anterior side of the tibia, 

 below the tibialis anticus, passes beneath a 

 strong restraining ligament, then lower down 

 beneath an osseous bridge, and lastly across 

 a strong ligament situated at the inferior ex- 

 tremity of the tarso-metatarsal bone. Below 

 this part its tendon divides into three slips 

 which are inserted into the distal phalanges 

 of the three outer toes (53). 



There are six long muscles lying on the 

 metatarsal bone ; they are largest and best 

 marked in those birds which walk most, as 

 the Aves terrestres. Two of these muscles 

 are on the posterior surface ; one goes to the 

 base of the external toe, which it abducts ; 

 the other is inserted into the root of the back 

 toe, which it bends. The other four muscles are 

 on the anterior part of the metatarsus : the first 

 extends the back toe; the second goes to the 

 base of the first toe, and abducts it; the 

 third is spread on the root of the middle toe, 

 which it extends ; the fourth lies along the out- 

 side of the metatarsus, perforates the end of 

 the bone, and is implanted into the inside of 

 the external toe, and abducts it. 



Progression on land is generally effected in 

 birds by the alternate advancement of the two 

 feet; but sometimes they proceed by leaping 

 or hopping, rather than walking ; both feet are 

 then firmly fixed on the ground, and the body 

 is propelled forwards by a sudden extension 

 of all the joints of the legs. Birds which have 

 sharp claws, as the Accipitres, Sj-c., retract them 

 when they hop, to prevent their being blunted. 

 The Cat tribe, among mammalia, have a me- 

 chanism effecting a similar purpose. Some 

 birds derive assistance in terrestrial progression 

 by the flapping of the wings, and this is 

 especially the case with the Ostrich, which 

 runs by the alternate advancement of its legs. 



The act of climbing is performed by means 

 of a peculiar disposition of the toes, aided by 

 prehension with the beak, as in the Maccaws 

 and Parrots, or by the prop formed by the stiff 

 tail-feathers, as in the Woodpeckers. 



The act of swimming is rendered easy 

 to birds by the specific levity of their body, 

 arising from the extension of the air-cells ; 

 by the shape of the chest, which resembles 

 the bottom of a boat ; and by the conversion 

 of the hinder extremities into oars in con- 

 sequence of the membranes uniting the toes 

 together. The effect of these web-feet in 

 water is further assisted by the toes, having their 



membranes lying close together when carried 

 forwards, whilst, on the contrary, they are ex- 

 panded in striking backwards. The oar-like 

 action of the hinder legs is still further favoured 

 by their backward position ; and by the meta- 

 tarsus and toes being placed almost on the same 

 perpendicular or vertical line with the tibia, an 

 arrangement, however, which is unfavourable 

 for walking. 



Sailing. Some birds, as the Swan, partially 

 expand their wings to the wind while swimming, 

 and thus move along the waters by means of 

 sails as well as oars. 



The act of diving is performed by the rapid 

 and forcible action of the wings, beating the 

 water as in flight, by the feet striking the waters 

 backwards and upwards, and assisted probably 

 by the compression of the air-cells. 



Flight, the most important and characteristic 

 mode of locomotion in birds, results principally 

 from the construction and form of the anterior 

 extremities, which have already been described. 



The form of the body has also especial 

 reference to this power, the tiunk being an 

 oval with the large end forwards. The spine 

 being short and inflexible, the muscles act 

 to great advantage, and the centre of gravity 

 is more easily changed from above the feet 

 as in the stationary position, to between the 

 wings as during flight. The head of the bird 

 is generally small, and the beak pointed, which 

 is a commodious form for dividing the air. 

 The long and flexible neck compensates for the 

 want of hands and the rigidity of the trunk, 

 and contributes to change the centre of gravity, 

 according to the required mode of progression, 

 by simply projecting the head forwards, or 

 drawing it back. The position of the great 

 pectoral muscles, as before observed, always 

 tends to keep the centre of gravity at the in- 

 ferior part of the body. The power which 

 birds enjoy of raising and supporting them- 

 selves in the air is undoubtedly aided by the 

 lightness of the body. The large cavities in 

 the bones diminish their weight without taking 

 away from their strength, a hollow cylinder 

 being stronger than a solid one of the same 

 weight and length. But the specific levity 

 principally depends on the great air-cells, which 

 occupy almost every part of the body, and 

 which are all in communication with the 

 lungs. The air which birds inspire distends 

 these cells, being expanded by the great heat 

 of the body. Lastly, the feathers, and especi- 

 ally the quills, from their lightness and elastic 

 firmness, contribute powerfully to the act of 

 flying by the great extent which they give to the 

 wings, the length and breadth of which are fur- 

 ther increased by the expanded integument 

 situated in the bend of the arm and in the 

 axilla. 



When a bird commences its flight it springs 

 into the air, either leaping from the ground, or 

 precipitating itself from some elevated point. 

 During this action it raises the humerus, and 

 with it the entire wing, as yet unfolded ; it next 

 spreads it horizontally by an extension or ad- 

 duction of the fore-arm and hand ; the greatest 

 extent of surface of the wing being thus acquired, 



