EFFERENT BRANCHIAL VESSELS. 257 



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 

 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 

 horizontal vessel. 



- From the ventral ends of the loops small arteries 

 arise, which supply the floor of the mouth and 

 adjacent 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 fine, 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 for- 

 wards 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 for- 

 wards 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. 



