684 DK. p. CHALMERS MITCHELL ON THE 



two, each branch again dividing into two, the four tendons 

 running respectively to the second digit, to each side of the third 

 digit and to the fourth digit. The ari-angement is practically 

 identical in Storks and Herons. 



Flexor perforans et 'perforatus {Flexor secundus) of digit II. — 

 Origin is from the external condyle of the femur distal to the 

 short arm of the biceps sling (text-fig. 129, Fl. 211, p. 680) and 

 from the fascia over the knee-joint, immediately superficial to the 

 corresponding flexor of the third digit with which it is closely 

 connected. The tendon passes in the normal fashion to the 

 second digit, perforating the tendon of the flexor primus and 

 being perforated by the branch of the tendon of the flexor 

 communis. 



Flexor jyerforans et perforatus {Plexor secundus) of digit III. — 

 This has two heads, one just deej) of the corresponding flexor of 

 the second digit and practically common with it, and a second 

 from the edge of the fibula. Its tendon receives a strong slip 

 from the tendon of the perforated flexor (flexor primus) of its own 

 digit and then is inserted to digit III in the same fashion as the 

 corresponding flexor of digit II. I did not record the existence 

 of the second head of this flexor in my notes on Storks and 

 Herons, but otherwise the flexores secundi have identical I'elations 

 in Bcdf&niceps, Stoi'ks and Herons. It must be noticed, however, 

 that these relations are found in a very large number of birds 

 belonging to difierent groups. 



Flexores perforati {Flexores primi) of digits II, III, IV; 

 Rudiment of A'iiibiens. — The perforated flexor muscles (text- 

 fig. 129, Fl. 1. II, Fl. 1. Ill, Fl. 1. lY) are very closely united. 

 Distally the tendons for the respective digits separate out ; 

 proximally the common muscular belly arises from three distinct 

 heads and the arrangement is such that fibres to each tendon can 

 be traced to each head. The largest head is fleshy from the 

 intercondylar notch of the femur ; there is an outer rather broad 

 tendinous head, superficial to the biceps tendon and arising from 

 the head of the fibula (text-fig. 129, Fl. 1. E). The third head is 

 a round and very distinct tendon, passing under the biceps tendon 

 and running partly to the head of the fibula and partly to the 

 fascia of origin of the flexores secundi (text-fig. 129, Amb.). The 

 inner fleshy head is normal and occurs in practically identical 

 form in all birds that I have dissected. The outei' tendinous 

 head also is usually present, and exists in Storks and Herons, the 

 chief difierences it presents being in the extent to Avhich it is 

 muscular. In Bcdceniceps, the tendinous portion is longer and 

 .the muscular portion relatively shorter than in Storks and Herons, 

 thus showing a degenerate condition. The rounded tendon under- 

 lying the biceps is more interesting. In birds where the curious 

 muscle known as the ambiens is present, the tendon of that 

 muscle passes through the knee-joint, passes under the biceps 

 tendon, sometiines with an anchor to the edge of the fibula, and 

 then forms a third head of origin of the perforated flexors. 



