218 ME. p. CHALMEKS MITCHELL ON SO-CALLED 



small and partly covered hj the biceps tendon, but frequently 

 possesses a fair proportion of muscular fibres. In all the 

 OLitaxic pigeous it is extremely small, in some of them being 

 practically reduced to a tendon. 



M. latiasimus dor si anterior et posterior. — These muscles are 

 extremely variable among birds, and pigeons show a considerable 

 range of difference. The general tendency among them is for 

 the anterior muscle, originally narrow and etrap-like, to extend 

 its origin and to broaden out ; the posterior, on the other hand, 

 is becoming reduced. In all the eutaxic pigeons, the anterior 

 muscle has a fairly broad origin ranging from about the third 

 last cervical, or just in front of that, to the first or second dorsal. 

 In this respect these pigeons cannot be said to be markedly in 

 front of the diastataxic forms, but they occupy an advanced 

 place among the progres^ive forms. On the other hand, they 

 are all markedly advanced in the degeneration of the posterior 

 muscle. In E there is a slight fibrous representative of it, in 

 the others it is completely absent. In diastataxic forms it is 

 frequently present, with distinct origin and insertion ; in Golumha 

 it is variable individually and specifically. 



Qrouf of Alar Muscles. — In the angle between the humerus 

 and forearm there are a number of muscular structures to which 

 great attention has been paid by Grarrod, Gradow, Fiirbringery 

 Beddard, and others ; and those structures have been shown to 

 possess considerable systematic value. In pigeons generally 

 there is great variety in the component parts of this group of 

 structures, and the differences both in their general aspect and 

 in their individual details have great significance. The general 

 tendency undoubtedly is to increase the musculature attached 

 nearer the proximal end of the forearm and to decrease that 

 more distally inserted. The mechanical eff'ect of these changes, 

 which obviously are progressive in the group, is to strengthen 

 the muscular pull, which from its insertion nearer the fulcrum 

 increases the rapidity of the upward movement of the forearm 

 on the humerus. In this general tendency towards change, a 

 more Passerine-like condition is being attained by many pigeons. 

 The eutaxic pigeons shov^ the changes, some of them in an 

 extreme form, all of them notably ; and in this respect it is in- 

 teresting to remember that the Passerines are a eutaxic group. 



Biceps patagialis. — This important patagial muscle is in origin 

 a slip from the biceps brachialis which runs to the long tendon 



