ARTERIAL SYSTEM. .531 



In Amphibia tlie piscine condition is most nearly retained*. 

 The mandibular artery is never developed, and the hyoid artery is 

 imperfect, being only connected with the cephalic vessels and never 

 directly joining the dorsal aorta. It is moreover developed later than 

 the arteries of the true branchial arches behind. The subclavian 

 arteries spring from the common trunks which unite to form the 

 dorsal aorta. 



In the Urodela there are developed, in addition to the hyoid, four 

 branchial arteries. The three foremost of these at first supply gills, 

 and in the Perennibranchiate forms continue to do so through life. 

 The fourth does not supply a gill, and very early gives off, as in the 

 Dipnoi, a pulmonary branch. 



The hyoid artery soon sends forward a lingual artery from its 

 ventral end, and is at first continued to the carotid which grows 

 forward from the dorsal part of the first branchial vessel. 



In the Caducibranchiata, where the gills atrophy, the following 

 changes take place. The remnant of the hyoid is continued entirely 

 into the lingual artery. The first branchial is mainly continued into 

 the carotid and other cephalic branches, but a narrow remnant of 

 the trunk, which originally connected it with the dorsal aorta, re- 

 mains, forming what is known as a ductus Botaili. A rete mirabile 

 on its course is the remnant of the original gill. 



The second and third branchial arches are continued as simple 

 trunks into the dorsal aorta, and the blood from the fourth arch 

 mainly passes to the lungs, but a narrow ductus Botaili still con- 

 nects this arch with the dorsal aorta. 



In the Anura the same number of arches is present in the 

 embryo as in the Urodela, all four branchial arteries supplying bran- 

 chiae, but the arrangement of the two posterior trunks is different 

 from that in the Urodela. The third arch becomes at a very early 

 period continued into a pulmonary vessel, a relatively narrow branch 

 connecting it with the second arch. The fourth arch joins the pul- 

 monary branch of the third. At the metamorphosis the hyoid artery 

 loses its connection with the carotid, and the only part of it which 

 persists is the root of the lingual artery. The first branchial artery 

 ceases to join the dorsal aorta, and forms the root of the carotid : the 

 so-called carotid gland placed on its course is the remnant of the gill 

 supplied by it before the metamorphosis. 



The second artery forms a root of the dorsal aorta. Tlie third, as 

 in all the Amniota, now supplies the lungs, and also sends off a 

 cutaneous branch. The fourth disappears. The connection of the 

 pulmonary artery with both the third and fourth branchial arches in 

 the embryo appears to me clearly to indicate that this artery was 

 primitively derived from the fou7-th arch as in the Urodela, and 

 that its permanent connection with the third arch in the Anura and 

 in all the Amniota is secondary. 



^ In my account of the Ampbibi.i, Gbtte (No. 296) has been followed. 



34—2 



