ARTERIAL SYSTEM 229 



the pharynx, behind the heart, to the corresponding point on 

 the opposite side. Turn the floor of the mouth, with the 

 heart, over to the right side. 



Wash the mouth and pharynx thoroughly. Dissect off the 

 mucous membrane of the roof of the mouth ; find the efferent 

 branchial vessels running inwards and backwards from the 

 gill-slits, and follow them, on the right side, outwards to the 

 gills, and inwards to the dorsal aorta in the middle line. 



The efferent branchial vessels form loops, one of 

 which runs round the margin of each of the first 

 four branchial clefts, and receives the arterial blood 

 returned from the gills of that cleft. A single vessel, 

 i.e. a half-loop, runs along the anterior border of the 

 fifth branchial cleft, and opens into the loop of the 

 fourth cleft. The successive loops communicate with 

 one another by short transverse branches about the 

 middle of their length. 



, From the ventral ends of the loops small arteries 

 arise, which supply the floor of the mouth and adja- 

 cent parts. 



From the dorsal ends of the loops, four main 

 efferent trunks, or epibranchial arteries, arise on 

 each side : these run backwards and inwards in the 

 roof of the mouth to the middle line, where they 

 unite together in pairs to form the median dorsal 

 aorta. 



a. The carotid artery is a small vessel which arises on 

 each side from the dorsal end of the efferent vessel 

 of the hyoidean gill, just in front of the origin of 

 the first efferent trunk. It runs forwards and 

 inwards across the hyo-mandibular cartilage, and 

 across the ventral surface of the skull opposite 

 the hinder border of the orbit ; and divides, about 

 one-third of an inch from the middle line, into 

 external and internal carotid arteries. Of these 

 the former enters and supplies the orbit, while 

 the latter, continuing its course in a groove in 



