212 NATURAL SCIENCE. Sept.. 



even in those cases where the first toe is absent there is present a 

 distinct flexor muscle corresponding in its position and relations to 

 the flexor longus hallucis, the long flexor of the great toe. The 

 tendon of this muscle has, of course, no great toe in which to be 

 inserted, and unites with the common tendon of the deep flexor 

 which splits up to work the other three toes. The writer pointed 

 out that the existence of this muscle appeared to be strongly con- 

 firmatory of Mr. Finn's view. Unless three-toed birds originally 

 were in possession of a hallux, why should they have a flexor 

 hallucis ? He proceeded to point out that this, however, cannot be 

 taken as the survival of a meaningless vestige. For, in the great 

 majority of avian groups (I accept Mr. Lucas's emendation, and 

 quite agree with him that the words of the note were expressed so 

 that a careless reader might suppose that individuals or species 

 were meant), the flexor tendon of the hallux, before running to 

 its insertion upon the hallux, gives off" a stout slip to the common 

 deep tendon of the other toes. Apparently it is this functional part 

 of the muscle that is preserved. Were there a case of a Passerine 

 where the hallux is absent, then we could see whether a functionless 

 rudiment would be retained ; for in Passerines there is no slip from 

 the longus hallucis to the common tendon of the other toes. 



Mr. Lucas brings forward interesting considerations directed to 

 the view that in birds there is a progressive tendency towards the 

 individualisation of these tendons. He suggests that, as is the case 

 in many existing Lizards, so possibly in primitive birds a single muscle 

 had many tendinous insertions, and that later on the muscle gradually 

 broke up till more and more of the tendinous insertions came to have 

 separate muscular bellies. He suggests, therefore, that in the case 

 of three-toed birds they may have lost their toe before the muscle 

 became individualised, and therefore he thinks that this condition of 

 the tendons means that the birds are low in the avian scale, and 

 therefore also, he concludes that the note does not supply an argument 

 in favour of Mr. Finn's view. The three-toed birds, according to him, 

 do not show evidence of the modification of their foot from a special- 

 ised four-toed foot, such as would be possessed by an arboreal form. 



The matter is so interesting that I shall elaborate the argument 

 more fully, as I am convinced that it is sound. In a typical four-toed 

 bird there are two deep plantar muscles. The belly of the flexor 

 longus communis lies most closely applied to the posterior surface of 

 the shaft of the tibio-tarsus. At its upper end it spreads out fan- 

 like, and takes origin sometimes continuously, sometimes by a series 

 of digitations, from the upper end of the tibio-tarsus, with occasionally 

 a slip from the femur. Its single tendon runs through a bony canal 

 in the posterior part of the ankle-joint, and then runs down the 

 posterior face of the shaft of the tarsus-metatarsus, near the lower end 

 of which it splits up into a tendon for the second, third, and fourth toes. 

 The flexor longus hallucis lies externally to the belly of the flexor 



