The Anatomy of Chlamydoselachus 



465 





a primitive condition since, in the early embryo, the dorsal aorta is paired throughout 



its entire length. As students of embryology know, the members of this pair of vessels 



meet in the median line, throughout the greater part of their length, to form the single 



dorsal aorta of adult anatomy. In gnathostomous vertebrates generally, the common 



carotid and the internal carotid arteries are regarded as anterior portions of the primitive 



dorsal aortae, which persist in the paired 



condition throughout life. These consid' 



erations lend interest to the study of these 



arteries in CMia'myd.osdach.us. My Text' 



figures 107 and 110, after Allis, will en' 



able the reader to follow the description 



of these arteries which I quote from Allis 



(1911, pp. 516-518) as follows: 



Running forward and slightly later- 

 ally, immediately beneath the broad and 

 rounded base of the chondrocranium, the 

 lateral aorta [ !da ] of each side is joined by 

 the corresponding efferent hyoidean artery 

 and then soon turns sharply laterally and, 

 at the edge of the base of the chondro- 

 cranium, receives the commissural vessel . . . 

 from the efferent hyoidean artery; this com- 

 missural vessel being considerably larger 

 than the lateral aorta. The latter vessel, 

 now becoming the common carotid, turns 

 sharply forward, at an acute angle, in the 

 direction prolonged of the commissural ves- 

 sel, runs forward and slightly mesially along 

 the lateral edge of the ventral surface of the 

 chondrocranium, and soon gives off its ex- 

 ternal branch. . . . 



The internal carotid, which is the an- 

 terior prolongation of the lateral dorsal 

 aorta beyond the point of origin of the ex- 

 ternal carotid, runs forward and mesially 

 along the base of the chondrocranium and, 

 not far from the median line, traverses a 

 foramen in the base of the skull and enters the cranial cavity 



Text-figure 109- 

 The carotid system of arteries in SqaoXui acanthias, 



ventral aspect. 

 ACA, anterior cerebral artery; APA, afferent pseudobranchial 

 artery; BA, buccal artery; CA, cerebral artery; CC, carotid 

 crossing; CCA, common carotid artery; CS, cephaUc sinus; DA, 

 dorsal aorta; DBCA, dorsal branchial commissural artery; EBA, 

 efferent branchial artery; £CA, external carotid artery; EHA, 

 efferent hyal artery; EPA, efferent pseudobranchial artery; ICA, 

 internal carotid artery; MCA, middle cerebral artery; OMA, 

 ophthalmic artery; PC A, posterior cerebral artery; PDA, paired 

 dorsal aorta; SA, segmental artery; SR, spiracular retia. 

 From Corrington, 1930, Text-fig. 22; after Hyrtl, 1872. 



Having entered the cranial 

 cavity, the internal carotid meets in the median line and anastomoses with, or is connected by 

 a short commissure with its fellow of the opposite side, and then immediately turns directly 

 laterally, and then forward and laterally in the cavity. There it is soon joined by the effer- 

 ent pseudobranchial artery, which artery enters the cranial cavity by traversing a foramen 

 in the orbital wall immediately anteroventral to the base of the eye-stalk. . . . Having 

 been joined by the efferent pseudobranchial artery, the internal carotid soon gives off an optic 

 branch and then separates into anterior and posterior cerebral branches, the latter of which 



