ARTEEIAL SYSTEM 233 



cmidal vein. Inject the caudal artery with plaster of Pa/ris, 

 coloured with carmine, and carefully strained so as to remove 

 cmy particles large enough to plug the vessels. Lay the ani/mal 

 in cold water for an hour. 



Insert one of the blades of a strong pair of scissors into the 

 mouth, and cut bach horizontally along the left side through 

 the gill-arches, as far back as the last gill-arch; from the 

 hinder end of this incision cut transversely across the floor of 

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

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

 hea/rt, over to the right side. 



Wash the mouth and pharynx thoroughly. Dissect off the 

 rmicous 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 margia 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. The successive loops communicate with 

 one another by short horizontal 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. 



Prom 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 firom the dorsal end of the efferent vessel 

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



