226 STRUCTURE AND CLASSIFICATION OF BIRDS 



the swifts a,nd humming birds. In Chmtura, Cypselus,^ and 

 Phaethornis and other humming birds the tensor brevis is 

 fleshy for almost its whole extent. In Dendrochelidon 

 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,^ 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 — . Gaerod, however, has left a 

 note to the effect that in Chmtura 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 Chmtura 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 

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

 this character may be gauged from the following table : — 



Without Biceps Sling , With Biceps Sling 



Chmtura spinicauda 



Chmtura caudacuta 

 Panyptila melanoleuca 

 Dendrochelidon coronata 

 Macropteryx mystacea 



Chmtura zonaris 

 Gypseloides fumigatus 

 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 



' Por Cypselus muscles see Nitzsoh-Giebel, ' Zur Anatomie d. Mauer- 

 •schwalbe,' Zeitschr. /. d. ges. Nat. x. 1857, p. 327. 



' This tendon looks like a degenerate representative of the abductor 

 pollioift. 



