ARTERIAL SYSTEM. 



647 



with the third arch in the Anura and in all the Amniota is 

 secondary. 



In the Amniota the metamorphosis of the arteries is in all 

 cases very similar. Five arches, viz. the mandibular, hyoid, and 

 three branchial arches are always developed (fig. 364), but, owing 

 to the absence of branchiae, 

 never function as branchial arte- 

 ries. Of these the main parts of 

 the first two, connecting the trun- 

 cus arteriosus with the collecting 

 trunk into which the arterial 

 arches fall, always disappear, usu- 

 ally before the complete develop- 

 ment of the arteries in the poste- 

 rior arches. 



The anterior part of the col- 

 lecting trunk into which these 

 vessels fall is not obliterated 

 when they disappear, but is on 

 the contrary continued forwards 

 as a vessel supplying the brain, 

 homologous with that found in 

 Fishes. It constitutes the internal 

 carotid. Similarly the anterior 

 part of the trunk from which the mandibular and hyoid arteries 

 sprang is continued forwards as a small vessel 1 , which at first 

 passes to the oral region and constitutes in Reptiles the lingual 

 artery, homologous with the lingual artery of the Amphibia; but 

 in Birds and Mammals becomes more important, and is then 

 known as the external carotid (fig. 125). By these changes the 

 roots of the external and internal carotids spring respectively 

 from the ventral and dorsal ends of the primitive third artery, 

 i.e. the artery of the first branchial arch (fig. 365, c and c'} ; and 

 thus this arterial arch persists in all types as the common carotid, 



FIG. 364. DIAGRAM OK THE AR- 

 RANGEMENT OK THE ARTERIAL 

 ARCHES IN AN EMBRYO OF ONE OF THE 



AMNIOTA. (From Gegenbaur ; after 

 RATHKE.) 



a. ventral aorta; a", dorsal aorta; 

 i, i, 3, 4, 5. arterial arches ; c. carotid 

 artery. 



1 His (No. 232) describes in Man two ventral continuations of the truncus arte- 

 riosus, one derived from the mandibular artery, forming the external maxillary artery, 

 and one from the hyoid artery, forming the lingual artery. The vessel from which 

 they spring is the external carotid. These observations of His will very probably be 

 found to hold true for other types. 



