MUSEUM OF COMPARATIVE ZOOLOGY. 213 



numerous small twigs, and the aortic roots plus the superior commis- 

 sures increase in importance with the retreat of the heart. In the Mam- 

 malia, where the aortic roots are reduced to the greatest extent, there 

 is a very great variety in the manner of origin of the carotids' from the 

 aorta, or, in other words, the primitive relation of the carotid trunks to 

 the median aorta has undergone a variety of transformations that for 

 the most part are characteristic of the groups in which they are found. 

 They remain as a pair of lateral longitudinal vessels, each of which 

 almost universally divides into two branches, a dorsal and a ventral. 

 The dorsal supplies the structures contained in the cranium, and gives 

 off vessels into the orbital space ; it is the internal carotid. The ven- 

 tral branch supplies the visceral portion of the head and the cranial pari- 

 etes, at least in part ; it is the external carotid artery. The external 

 carotid is then only a ventral branch of the dorsal commissural trunk, 

 while the internal carotid continues the main stem of the common 

 carotid. Of course, the relative sizes of the vessels undergo ceaseless 

 variations as we ascend the series ; but the rule is, that the higher the 

 position of the animal in the series, the more important relatively the 

 territory supplied by the external carotid becomes. For example, in 

 man the two vessels, internal and external, are of about the same size ; 

 in the cat, the internal is small and the external correspondingly large. 

 The two vessels may be united into a common trunk, and always are 

 when the aortic arch from which they arise is much reduced, or they 

 may arise independently of one another,' as in Myxine. So long as the 

 dorsal aorta persists entire, the carotids have no existence ; but just in 

 proportion as the precardiac section of the aorta is reduced, the carotid 

 arteries become more and more important, until they ultimately entirely 

 replace it, as in the vertebrates above the lower fishes. In Myxine, 

 " Aus dem Zusammenflus der Kiemenvenstamme entstehen vier Haupt- 

 arterienstamme fur den Korper, ein vorderer und hinterer unpaarer 

 mittelerer, welche unter der Wirbelsaule hingehen, und zwei seitliche 

 vordere. Die vorderen Theile des Kbrpers besitzen also zwei Carotiden 

 und eine unpaare Wirbelarterie [i. e. precardiac Aorta] die hintern 

 Theile des Korpers einen einzigen Arterianstamm, die Aorta descendens. 

 Die Kiemenvenen der zwei oder drei letzten Kiemen gehen direct in die 

 nach vorn und unten gleich sich verlangernde Aorta. Die Kiemenvenen 

 der ersten oder zwei ersten Kiemen gehen nicht mehr in die Aorta iiber, 

 sondern vereinigen sich jederseits in eine der Aorta parallele vena bran- 

 chialis communis, welche sich nach vorn als Carotide fortsetzt. Die 

 directe Fortsetzung der Aorta nach vorn, verlauft als arteria vertebralis 



