93 
Now the area of each lamella can be approximately caleu- 
lated, since it is nearly triangular. It is roughly 0°365 
square millimetre. But since both the flat surfaces of the 
lamella are in contact with the water, 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 } 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. em., 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 afferent 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 (A. ep.) unite to form 
the dorsal aorta; anteriorly they are connected 
together by a short anastomosing vessel (C2. ¢.). 
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 be 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 
