212 BULLETIN OF THE 



terior,"* as correctly describing the morphological relations of the ves- 

 sels. Now, in as far as the terms are applied to the carotid arteries of 

 the higher vertebrates as names simply, it matters little which set is 

 used ; but when it is proposed to select terms that shall harmonize with 

 the development of the vessels f under consideration, superior and infe- 

 rior are undoubtedly the correct ones, or dorsal and ventral. 



In the light of Chlainydoselachus we may reasonably choose to retain 

 the terms internal and external as describing at the same time the 

 primitive condition of the vessels in the lowest vertebrates, and, when 

 we consider the relations of the vessels to their territory of distribution 

 (their only constant relation), and not alone their origin (in the anatomi- 

 cal sense) and source of supply (their constantly varying relation), also 

 their anatomical relations up to and including man. The vessels are 

 really never anterior and posterior so far as their points of origin are 

 concerned, and this, as I understand it, is the basis of Parker's deter- 

 mination. As shown above, in primitive forms (e. g. Amphioxus and 

 Myxine), the region supplied by the internal carotids in Acraniates 

 and Craniates is provided for by branches from the superior portion of 

 the curve of the first pair of aortic arches. The carotids arise from 

 the dorsal prolongations of the aortic roots ; i. e. from the tract ho- 

 mologous with the superior commissures of Elasmobranchs, and not, as 

 Macalister and others suppose, the anterior ends of the two lateral 

 aortse on the one hand, or the corresponding parts of the ventral aortae 

 on the other. The carotid arteries are, in a strict sense, separated from 

 the aorta by the vessel crossing the space between the dorsal end of the 

 gill and the aorta which lies in the middle line. This vessel is equal to 

 the dorsal portions of the efferent branchial arteries (or the entire epi- 

 branchials of Parker). It is because the inferior commissures are 

 merely passageways for the transmission of blood to the distributing 

 vessels in the dorsal region of demand, that they do not persist, since 

 their function is early assumed by other vessels. As the aorta is gradu- 

 ally reduced by the backward journeying of the heart consequent on the 

 reduction of the branchial vessels and organs, the brain and the enclos- 

 ing head are removed farther and farther from the aortic arches from 

 which they originally received their blood supply direct, by means of 



* Parker's A. carot. post. = Art. carot. int. 



" " ant. = eff. br. art. of mandibular gill, and both these ves- 



sels supply the region of the external carotid in sharks. 



t Cf. Rathke (20) or Balfour (24). Corap. Embryol, II., 1881. I refer here to 

 the ilevelopment in the higher vertebrates. 



