XIII 



PHYLUM CHORDATA 



.31)5 



raising tlie humerus. There are three tensores ivitagii (//is. ///., ///.s\ 

 b/\, ins. ace), the action of which is to keep the pre-patagium tensely 

 stretched when the wing is extended. A similar muscle (tns. m.ii). 

 acts upon the post-patagium. The muscles of the digits are 

 naturally much reduced. 



The muscles of the neck and tail are well developed ; those of 

 the back are practically atrophied, in correspondence with the iin- 

 mobility of that region. lu the leg certain of the muscles are 

 modified to form the perrhiiuj nicrhxnlsm. The toes are Hexed 

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

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



Cor.hr.lti 

 ab.cL f^ 



sb.clv 



irn.hr 7„ 



^rrt 



Fio. 1030. — Columba livia. The principal muscles of the left wing ; the greater part of tlic 

 pectoralis f/«•^) is removed. c(it. st. carina sterni ; cl. furcula ; cor. coracoid ; cor. hr. br. coraco- 

 brachialis lire vis ; cor. br. Ig. coraco-brachialislongus ; cp. st. corpus sterni ; ext. cp. rd. extensor 

 cai-pi radialis ; ext.rp. ul. extensor carpi uliiaris ; /. cp. vl. flexor corpus sterni ; gl. c. glenoid 

 cavity ; liu. head of humerus ; Icu'. its distal end ; /id. pectoralis ; pet', it.s cut edge ; pet", its 

 insertion ; prn. hr. pronator brevis ; prii. Ig. pronator longus ; pr. ptgm. pre-patagium ; 

 pt. ptgm. post-patagium ; sb. civ. subclavius ; .s'j. ctv'. its tendon of insertion passing through 

 the foramen triosseum, and dotted as it goes to the humerus ; tns. ace. tensor accessorius ; tns. 

 hr. tensor brevis ; tii.i. Ig. tensor longus ; tns. m. p. tensor membranaj posterioris ala;. 



trifurcation of the tendon of a single muscle, the ijeronceui^ medius, 

 that of the hallux is derived from a separate muscle, the flexor 

 ■perforans, which is joined by a slip from the peronfeus 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 is asleep by the mere weight of the body. The action 

 is assisted by a small but characteristic muscle, the ambiens, which 



