Chap. X.] THE HEAR7" AND CIRCULATION. 203 



arch is well developed below, but gives rise to a large posterior 

 pulmonary branch running to the lung. The dorsal connection 

 with the epibranchial (systemic arch) is slender. The fourth 

 arch is incomplete above, but joins by a slender tube (in the 

 newt if not in the frog) the pulmonary branch, of which pul- 

 monary branch the fourth arch is indeed most probably the 

 true parent, the connection with the third arch being subse- 

 quent. Fig. 62, in., shows the state of things in the adult frog. 

 The hyoid arch has atrophied, and the lingual artery derived 

 from it has become connected with the carotid. The slender 

 connection between the carotid and the systemic aorta has 

 atrophied, as has also the dorsal connection of the third arch, 

 and the whole of the fourth. Thus in the adult frog 



The lingual artery is derived from the hyoidean arch. The 

 carotid artery is derived from the first branchial arch. The 

 systemic aorta is derived from the second branchial arch. The 

 pulmo-cutaneous artery is believed to be derived partly from 

 the third branchial arch, and partly from the fourth branchial 

 arch. 



We may now consider, with the aid of Fig. 62, iv., how the 

 heart works. The truncus arteriosus, ventricle, and auricles 

 are partially laid open. By the contraction of the atrium 

 venous blood from the sinus venosus (s. v.) and right auricle, 

 and arterial blood from the pulmonary vein (p. v.) and left 

 auricle, are received simultaneously into the cavity of the 

 ventricle (v.). By the contraction of the ventricle venous and 

 arterial blood pass successively into the truncus arteriosus. 

 The longitudinal swing-valve in the truncus arteriosus is so 

 arranged that blood passing to the pulmo-cutaneous orifice 

 (p. c.\ passes to the left, while blood passing to the carotid (ca.) 

 and systemic (s. ao.) orifices passes to the right. 



Now since 



1. The opening into the truncus arteriosus is well over to 



the right side of the ventricle ; 



2. The blood in the ventricle is arterial to the left and 



venous to the right, the two not completely mixing 

 owing to the spongy texture ; 



