FEB < 



1893 



No. 2. — Note on the Carotids and the Ductus Butalli of the 

 Alligator. By C. B. Davenpokt.^ 



The carotids of the Alligator, as is well known, usually present in 

 the adult an unsyrametrical condition. From the right aortic root two 

 branches (« and h) arise close together. Of these, one goes to the left, 

 the other to the right side of the body. The left vessel (a) gives rise 

 first to a large artery (1) running on the dorsal side of the oesophagus 

 to the head ; secondly, to a smaller one (2) running alongside the neck, 

 sending branches to the oesophagus, trachea, and muscles of the body 

 wall, and anastomosing behind the occiput with a branch of 1 ; and, 

 thirdly, to a vessel (3) going to the fore limbs. Other smaller vessels 

 we may for present purposes neglect. The right vessel (b) gives rise to 

 all that the left does excepting the equivalent of 1. 



Concerning the homology of these vessels with those of other Eeptiles 

 and of Birds, there has been much difference of opinion. Rathke ('57, 

 p. 91) designated the vessels a and b arteria anonyma. He called 1 

 art. subvertebralis, and from embryological data he interpreted it (as 

 defined in the preceding paragraph) as follows. The right and left com- 

 mon carotids arise separately, and later fuse along the middle of their 

 extent, but remain separate at their proximal and distal ends. Secon- 

 darily, the right proximal root degenerates, and is thus absent in the 

 adult. 



By " common carotid," Rathke implies an homology with a vessel of 

 this name in Lizards. The latter is morphologically equivalent to the 

 ventral distributing trunk between the third and fourth arterial arches in 

 Rathke's well known type scheme of the arterial arches. The nnfused 

 distal ends of the common carotids each branch at the occiput to form 

 vessels homologous, in his opinion, to the internal and external carotids 

 of Mammals. 



Fritsch ('69, p. 705) later maintained that the a. subvertebralis is an 

 a. carotis primaria, i. e. produced by a drawing out of the a. anonyma, 



1 Contributions from the Zoological Laboratory of the Museum of Comparative 

 Zoology, under the direction of E. L. Mark, No. XXXIV". 



VOL. XXIV. — NO. 2. 



