" QUINTOCUBITALISM " IN THE WING OF BIRDS. 221 



longus tendon arises almost directly from the general muscle. 

 As for the differentiation of the brevis tendon of insertion into 

 ' alpha,' 'beta,' and ' gamma,' it occurs in A, a, B, E, and F, the 

 pectoralis slip forming a considerable portion of ' beta.' In the 

 others the normal condition is attained, not passed. 



'Pectoralis slip.— In the simple pigeon-condition, as in fig. 2, 

 Pec, the pectoralis muscle gives off two slips partly muscular 

 and continuous at origin, and these run in altnost equal propor- 

 tions to the longus and brevis tendon. The general tendency is 

 for the longus slip to weaken or disappear, and for the brevis 

 slip to become an exceedingly sharply-marked round tendon 

 passing under the deltoides patagialis, and ultimately forming 

 part of the ' alpha ' or 'beta' tendon. This ultimate condition 

 recalls that common in Passerines, where there is only a single 

 tendon running to the brevis. The longus part of the patagial 

 is absent in B, C, D, and very short and slight in the others. 



Deltoides major. — The series of progressive changes found 

 among pigeons in this muscle are of very great significance and 

 interest. They concern its insertion, size, and subdivision. 

 Typically, the muscle is moderately well developed and is inserted 

 for a varying distance down the humerus. In pigeons there are 

 two grades in its development. In the first grade, as Fiirbringer 

 showed, the muscle tends to increase the length of its insertion 

 until ultimately it reaches almost to the distal end of the 

 humerus, w^here it may be pierced by the large nerve which 

 supplies the forearm. This condition, represented in fig. 4 I, is 

 shown at its maximum in most of the specimens of Columha livia 

 which I have dissected, but in some specimens it has not been 

 reached. Among the eutaxic pigeons it occurs in B. In the 

 others, a second series of changes begin. In D the muscle is 

 completely divided into two portions. The upper and smaller 

 portion is inserted to the humerus near the distal end of the 

 supracoracoideus muscle-tendon. In A, a, C, E, and E this 

 first part is similarly separated off". The second part in D reaches 

 down the humerus in the usual fashion, but is not actually 

 divided by the radial nerve. In A and E the second part has 

 reached right down the humerus and is pierced b)^ the radial 

 nerve. It is, however, very thin and attenuated, while the 

 deltoides patagialis and anconseus are both unusually large. In 

 E (fig. 4, II) the conditions are similar, but that part of the 

 attenuated long division of the muscle which lies next the 



