30 THE VASCULAR SYSTEM OF THE FROG 



Dissect as for the veins. Pass a roll of paper or plugs of cotton- 

 wool down the oesophagus, so as to distend it and stretch the aortic 

 arches. Clean carefully the aortic arches, commencing at the trun- 

 cus arteriosus; and follow the several arteries to their distribution, 

 removing the veins and other structures which overlie them. Note 

 the division of the trunciis arteriosus in front into right and left 

 branches, each of which again divides into three aortic arches the 

 carotid arch, the systemic arch, and the pulmo-cutaneous arch. 



I. The Carotid Arch is the most anterior of the three 

 arches : it runs round the side of the oesophagus, and is 

 connected dorsally with the second or systemic arch; its 

 chief branches are as follows : 



1. The external carotid artery is a small artery supply- 



ing the tongue. Immediately beyond the origin of 

 the lingual artery the carotid arch presents a small 

 spongy swelling, the carotid gland. 



2. The internal carotid artery runs round the side of 



the oesophagus to its dorsal surface : it is connected 

 with the systemic arch by a short branch, the ductus 

 Botalli, which in the adult frog is usually impervious ; 

 and then turns forwards beneath the base of the skull, 

 dividing in front into the two following vessels : 

 i. The palatine artery, supplying the roof and sides 



of the buccal cavity, and the orbit, 

 ii. The cerebral artery, which enters the skull and 



supplies the brain. 



II. The Systemic Arch, the middle arch of the three, runs 

 somewhat obliquely round the oesophagus to the dorsal surface, 

 and unites with its fellow of the opposite side about the level of 

 the anterior ends of the kidneys. From this point the right 

 arch gives off the main artery to the viscera, the left arch 

 being the chief source of the dorsal aorta. Near the level of 

 the posterior ends of the kidneys the aorta divides into the two 

 i \ fop. Arteries. The branches of the systemic arch are as follows : 



a. Branches given off before the union of the two arches. 



1. The laryngeal artery is a small branch arising from the 



inner side of the systemic arch near its origin from 

 the truncus arteriosus, and supplying the larynx. 



2. The oesophageal arteries are one or two branches arising 



