Pseiidobranchial and Carotid Arteries in the Guathostome Fishes. 129 



In the Anura the vessels are doubtless similar to those in the 

 Urodela, but Hochstetter gives no figure of them. 



Summary. 



The encephalic arteries of the Gnathostomata supply blood to 

 the brain, and they probably primarily formed a direct anterior 

 prolongation of the lateral dorsal aorta of either side beyond the 

 point Avhere that vessel is joined by the dorsal end of the mandi- 

 bular aortic arch. 



These vessels, in young embryos of elasmobranchs, receive their 

 ■entire blood supply through the mandibular aortic arch, and in this 

 apparently primitive condition, the encephalic artery of either side 

 forms the entire internal carotid, properly so-called, of the fish. 

 .But, in further development, the tendency is to establish a forward 

 current in the anterior portion of the lateral dorsal aorta, this 

 current at first simply supplementing, but later wholly replacing, 

 the current through the mandibulai- arch. This dorsal-aortic current 

 is first derived through the hyoidean aortic arch, then, by means 

 of a commissural connection, through that arch and the glosso- 

 pharyngeal arch, and finally wholly through the latter arch. 



When the blood current to the encephalic artery is derived 

 wholly or mainly through the mandibular aortic arch, the efferent 

 mandibular and encephalic arteries together form what has been 

 called an anterior internal carotid ; and when the current is derived 

 through the dorsal aorta, the section of that artery so concerned, 

 together with the encephalic artery and a certain varying portion 

 of the hyoidean oi- glossopharyngeal aortic arches, is usually called 

 the posterior or the common-plus-internal carotid. And in either 

 form of development the tendency is to restrict the current to the 

 aortic arch concerned, b}^ the reduction, or abortion, of that section 

 of the lateral dorsal aorta that connects that arch with the next 

 posterior one. 



There are accordingly three types of internal carotid: a 

 mandibulo-internal, a hyo-internal, and a glosso internal, and in 

 adult vertebrates, either one of these types may be found dominant, 

 and either of the last two absolute. 



In adult chondrostean ganoids the mandibulo-internal type of 

 carotid is strongly dominant. In adult elasmobranchs (excepting 

 Torpedo) the type is mixed; the nomenclature used in descriptive 



Zool. Jahrb. XXVII. Abt. f. Anat. i 9 



