188 THE MYOLOGY OF THE KAVEN. 



minimus, and tlie vastus externus muscle of the exten- 

 sor femoris is inserted on a longitudinal line between 

 them (Fig. 49). In the absence of the accessory femoro- 

 caudal as we find it here, the sciatic nerve and artery 

 passes over and external to the tendon of the obturator 

 externus, its track being impressed upon it in spirit 

 sj)ecimens. 



In the course of my remarks on the value of certain 

 of these muscles in the classification of birds, published 

 in another connection (see 124 of Bihliography) I said 

 that, " There are Jive muscles in the thigh which have 

 proved to be more or less useful in the classification of 

 birds. These muscles are the following, and four of 

 them I have designated by the letters which were used 

 by Garrod in his myological formulae. 



6. The ambiens, 



7. The femoro-caudal A 



8. The accessory femoro-caudal B 



9. The scmitoidmosus X 



10. The accessory seiyiitendinosus Y 



" AVe know of no bird in which all five of these 

 muscles are absent, or' even of one which lacks the last 

 four in the list. 



"According to Garrod, 'when these four muscles are 

 present in a bird, the formula AB. XY expresses the 

 fact ; when any one is absent, that such is the case is 

 indicated by the omission of the letter representing it. 

 Thus the formula A. XY indicates that the accessory 

 femoro-caudal muscle only is absent ; AB. X that the 

 accessory semitendinosus is missing; A.X that the 

 femoro-caudal and semitendinosus only are to be found ; 

 and A that the femoro-caudal alone is present.' 



