EFFERENT BRANCHIAL VESSELS. 249 



branchial vessels running imvards 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 eiferent branchial vessels form loops, one of 

 which runs round the margin of each of the first 

 four branchial clefts, and receives the arterial blood 

 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. Each loop communicates with the next 

 one about the middle of its length by a short hori- 

 zontal vessel. 



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 

 eiferent 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 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 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 runs forwards and outwards 

 across the floor of the orbit, and supplies the 

 upper jaw and snout ; while the latter, continuing 

 its course in a groove in the ventral surface of 

 . the skull, passes through a median foramen into 

 the cranial cavity. In the foramen the two 

 internal carotids cross each other, each uniting 

 within the cranial cavity with the hyoidean 

 artery of the opposite side. 



