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MR. A. H. GARROD ON THE 



[May 6, 



of the thorax it divides into the carotid, vertebral, and thyroid 

 branches, except in those in which the carotid of one side is deficient. 

 In what may be called the typical arrangement, the carotids, equal 

 in size or nearly so, run up the front of the neck from the inner side 



Fig. 1 



Fig. 2. 



T.S 



Fig. 1 . Carotids at the base of the neck in uvea hiearofidina nor males. 

 Fig. 2. Carotids at the base of the neck in aves lavo-carotidince. 



of each thyroid gland, converging until they meet in the middle line, 

 at which spot they enter the median intermuscular septum, and con- 

 tinue up to the head, on the front of the bodies of the remaining 

 cervical vertebrae, in the hypapophysial canal, covered by the lateral 

 cervical muscular masses, and, where they are present, threading 

 the bony arches. Birds with this arrangement are said to have two 

 carotids, and may be termed aves bicarotidince normales (see fig. 1). 



A second group is peculiar in having the right carotid branch of the 

 innominate undeveloped, when the left only traverses the hypapo- 

 physial canal, being of large size ; it bifurcates shortly before it 

 reaches the head, thus producing a vessel on each side, to be 

 distributed in the same way as the terminations of the carotids in the 

 previous group. Such birds are said to have a left carotid, and 

 may be termed aves Icevo-carotidince (see fig. 2). 



In a third arrangement, found only in certain Parrots (see fig. 3), 

 the right carotid artery runs in the hypapophysial canal, and the 

 left at the side of the neck superficially along with the corresponding 



* In these diagrams, which represent the main arteries at the root of the neck, 

 the following is the explanation of the abbreviations : — h, origin of the aorta at 

 the heart; a. arch of the aorta ; l.i, left innominate artery ; r.i, right innominate 

 artery ; l.s, left subclavian, and r.s, right subclavian artery ; l.e, left carotid, and 

 r.c. right carotid arterv. ' 



