226 STRUCTURE AND CLASSIFICATION OF BIRDS 



the swifts and humming birds. In Chcetura, Cypselus, 1 and 

 Phaethornis and other humming birds the tensor brevis is 

 fleshy for almost its whole extent. In Dendroclielidon 

 the tensor brevis has still a larger muscular portion than is 

 usual, but the tendon is more evident and has a passerine 

 slip to the humerus. In the Trochili, however, the muscle 

 is inserted on to a special tendon upon the fore arm, 2 and 

 not on to the extensor metacarpi. In the leg muscles 

 the birds of this group agree in only possessing the femoro- 

 caudal of those used by GAEEOD in his classification ; the 

 formula, therefore, is A . GARROD, however, has left a 

 note to the effect that in Ch&tura caudacuta the femoro- 

 caudal passes through a muscle arising from both pubis 

 and ischium, which is thus possibly a combined semitendi- 

 nosus and semimembranosus. 



Though the semitendinosus is as a rule absent, there 

 seem to be traces of its accessory in a few swifts. Thus 

 in Cypselus alpinus and Chcetura Vauxi the gastrocnemius 

 has an origin between the biceps loop and the main body 

 of the sciatic nerve from the femur. 



Another peculiarity in the leg of certain swifts (cf. also 

 Phaethou) is the absence of a biceps loop ; but the value of 

 this character may be gauged from the following table :- 



Without Biceps Sling 



Chcetura caudacuta 

 Panyptila melanoleuca 

 Dendrochelidon coronata 

 Macropteryx mystacea 



AVith Biceps Sliny 



Chcetura spinicauda 

 Cluetura zonaris 

 Cijpseloides fumic/a tit* 

 Cypselus alpinus 



The biceps femoris of humming birds at any rate of 

 Patagona gigas is peculiar in the fact of its being two- 

 headed. 



The deep flexor tendons in the swifts vary. In the 

 majority of forms the two tendons completely blend ; in 



1 For Cypselus muscles see NITZSCH-GIEBEL, ' Zur Anatomie d. Mauer- 

 schwalbe,' Zcitschr. f. d. ges. Nat. x. 1857, p. 327. 



2 This tendon looks like a degenerate representative of the abductor 

 pollicis. 



