532 



ARTERIES OF THE A MX I OTA. 



In the Amtiiuta the tnetamorphosis of the arteries is in all ea^es 

 very similar. Five arches, viz. tlie mandihular, hyoid, and three 

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



absence of branchiie, never 

 function as branchial arteries. 

 Of these the main parts of 

 the first two, connecting the 

 truncus arteriosus with the 

 collecting trunk into which 

 the arterial arches fall, always 

 disappear, usually before the 

 complete development of the 

 arteries in the posterior arches. 

 The anterior part of the 

 collecting trunk into which 

 these vessels fall is not ob- 

 literated when they disappear, 

 but is on the contrary con- 

 tinued forwards as a vessel 

 supplying the brain, homo- 

 logous with that found in 

 Fishes. It constitutes the in- 

 ternal carotid. Similarly the 

 anterior part of the trunk from which the mandibular and hyoid 

 arteries sprang is continued forwards as a small vessel^ which 

 at first passes to the oral region and constitutes in Reptiles the 

 lingual artery, homologous with the lingual artery of the Am- 

 phibia; but in Birds and Mammals becomes more important, and 

 is then known as the external carotid (fig. ] 25). 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. 3(55, c and c') ; and thus 

 this arterial arch persists in all ti/pss as the common carotid, and the 

 basal part of the internal carotid. '1 he trunk connecting the third 

 arterial arch with the system of the dorsal aorta persists in some 

 Reptiles (Lacertilia, fig. 366 A) as a ductus Botalli, but is lost in 

 the remaining Reptiles and in Birds and Mammals (fig. 366 B, C, D). 

 It disappears earliest in Mammals (fig. 365 C), later in Birds (fig. 

 365 B), and still later in the majority of Reptiles. 



The fourth arch always continues to give rise, as in the Anura, to 

 the system of the dorsal aorta. 



In all Reptiles it persists on both sides (fig. 366 A and B), but with 

 the division of the truncus arteriosus into three vessels one of these, 



Fig. 364. Diaokam of the arrangement 

 or the arterial arches in an embryo of one 

 OF THE Amniota. (From Gegenbaur ; after 

 Kathke.) 



a. ventral aorta; a", dorsal aorta; 1, 2, 

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



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

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

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

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

 be fo'uid to hold true for otl.e.' types. 



