AORTIC ARCHES OF BIRDS — GLENNY 543 



blood supply while the right vessel was reduced to a ligamentum 

 ottleyi. Both dorsal carotids have been found to be present as 

 ligamenti ottleyi in Bucorvus abyssinicus and in Rhopodyles mridirosiris. 



Another variation in the arrangement of the dorsal carotids results 

 from the superficial position of one of these vessels while the com- 

 plimentary vessel lies within the hypapophysial canal. This is 

 observed most commonly among the Psittaciformes, in which order 

 the right carotid enters the hypapophysial canal while the left carotid 

 is superficial and lies in close association with the vagus nerve of that 

 side. 



Some of the aberrancies noted among related species and genera 

 emphasize the importance of geographical and ecological distribution 

 of species, with the resultant specific and subspecific isolation as 

 factors in the selection of successful types which may be found to 

 present these anatomical variations. In addition to other factors, 

 anatomical variations may, in conjunction with studies of geographical 

 distribution, serve to show more clearly possible lineage within a 

 family or order of birds on the one hand and possible routes of move- 

 ment and dispersal in the course of evolution on the other hand. 



The exact site of origin of the coracoid or sternoclavicular artery 

 varies somewhat in different families of birds. Generally this vessel 

 is found as a branch of the subclavian just medial to the axillary 

 artery, but in a few orders it arises from different points on either the 

 subclavian or the pectoral stem, and in some instances two, or rarely 

 three, pairs of these vessels are present. In order to facilitate the 

 classification of these vessels the following scheme is proposed: 



Type A: coracoid artery is medial to the axillary. 



Type B: coracoid artery is opposite the base of the axillary. 



Type C: coracoid artery is lateral to the axillary. 



Type D: two coracoids are present; one is medial or opposite the base of the 



axillary, the other is lateral to the axillary. 

 Type E: two coracoids are present; both are medial or opposite the base of the 



axillary. 

 Type F: two coracoids are present; both are lateral to the base of the axillary. 



The thoracic, intercostal, or internal mammary artery of birds 

 likewise is found to arise at slightly different relative positions — from 

 a point at the base of the inferior pectoral artery to a point near the 

 base of the coracoid or sternoclavicular artery, and in some instances 

 both of these vessels have a common root from the subclavian artery. 

 Such differences are found to be of common occurrence within several 

 orders of birds. In the Galliformes and the Passeriformes there 

 appears to be a graded series in the sites of attachment of the thoracic 

 artery from a lateral to a medial position. As a result of these obser- 

 vations, numerical values can be assigned to the site of attachment of 

 the intercostal or thoracic artery, and these values may come to be 



