xin 



PHYLUM CHORDATA 



383 



by two sets of tendons, deep and superficial. The deep tendons 

 of the three forwardly-directed digits are formed by the trifurcation 

 of the tendon of a single muscle, the peronceus medius ; that of the 

 hallux is derived from a separate muscle, the flexor perforans, 

 which is joined by a slip from the peronseus medius. Thus a 

 pull upon one tendon flexes all the toes. When the leg is bent, 

 as the bird settles to roost, the flexion of the tarso-metatarsus 

 on the shank puts the flexor tendons on the stretch as they pass 

 over the mesotarsal joint, and by the pull thus exerted the toes 

 are automatically bent round the perch by the simple action of 

 flexing the leg. They are kept in this position while the Bird 



FIG. 1050. Columba livia. The principal muscles of the left wing ; the greater part of the 

 pectoralis (pet.) is removed, car. st. carina sterni : c7. furcula ; cor. coracoid ; cor. br. br. 

 coraco-brachialis hrevis ; cor. br. Iff. coraco-brachialis longus ; cp. st. corpus sterni ; ext. 

 cp. rd. extensor carpi radialis ; ext. cp. ul. extensor carpi ulnaris ; fl. cp. ul. flexor carpi 

 ulnaris; gl. c. glenoid cavity ; hu. head of humerus ; hu'. its distal end ; pet. pectoralis ; 

 pet', its cut edge ; pet", its insertion ; prn. br. pronator brevis ; prn. Irj. pronator longus ; 

 pr. ptgm. pre-patagium ; pt. ptgm. post-patagium ; sb. civ. subclavius ; sb. civ', its tendon 

 of insertion passing through the foramen triosseum, and dotted as it goes to the humerus ; 

 tns. ace. tensor accessorius ; tns. br. tensor brevis ; ins. Ig. tensor longus ; tns. m. p. tensor 

 membrante posterioris ate. 



is asleep by the mere weight of the body. The action is assisted 

 by a small but chafacteristic muscle, the ambiens, which arises 

 from the pubis, passes along the inner surface of the thigh, and 

 is continued into a long tendon which comes round to the outer 

 side of the knee, enclosed in a special sheath, and, continuing 

 down the leg, joins the superficial flexors of the digits. 



Digestive Organs. The mouth (Fig. 1051) is bounded above 

 and below by the horny beak, and there is no trace of teeth. 

 The tongue (tng.) is large and pointed at the tip. The pharynx 

 leads into a wide and distensible gullet (gul.) which soon dilates 

 into an immense reservoir or crop (crp.) situated at the base of 

 the neck, between the skin and the muscles, and immediately in 



