93 



Now the area of each lamella can be approximately calcu- 

 lated, since it is nearly triangiilar. It is roughly 0'365 

 square millimetre. But since both the flat surfaces of the 

 lamella are in contact with the Avater, the respiratory sur- 

 face is double this, and is 0-730 sq. mm. x 349,200 = 254,916 

 sq. mm. That is over ^ square metre. The total respira- 

 tory surface of the gills is therefore that of a .square, the 

 length of the side of which is j metre. These calculations 

 apply to a plaice of about 22 inches long. The area of the 

 skin of such a fish is approximately 2,340 sq. cm., or 

 nearly ^ sq. metre. The respiratory surface of the gills is 

 therefore about equal to the total area of the skin. 



The Efferent Branchial Vessels. — The blood, after 

 having passed from the heart and att'erent vessels through 

 the lamellar capillaries, is collected by four trunks on 

 each side — the efferent branchial vessels. These open 

 into the epibranchial arteries of each side. Posteriorly 

 the two epibranchial arteries (^4. ep.) unite to form 

 the dorsal aorta ; anteriorly they are connected 

 together by a short anastomosing vessel (Cir. c). 

 The loop thus formed is the circulus cephalicus. It is the 

 reservoir into which the blood, after having undergone 

 oxygenation in the gills, is poured, and from which it is 

 distributed over the body. The efferent branchial system 

 is best injected from the dorsal aorta after tying the 

 coeliaco-mesenteric artery. It can be displayed after 

 cutting away the greater portion of the operculum of one 

 side, removing the opercular, sub-opercular and inter- 

 opercular bones. The remaining dorsal portion of the 

 operculum is then forced outwards and held in position by 

 a hook. The gill filaments should b,e cut away close to 

 the arches. The vessels themselves are then seen, after 

 dissecting apart and removing most of the muscles, pass- 

 ing dorsally from the gill arches. The circulus cephalicus 



