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which also receives, through a special canal, the blood from 
the digestive organs. This antechamber communicates directly 
with the two-chambered heart, which corresponds — in function, 
at any rate — with the right half of the four-chambered heart of 
warm-blooded animals. The blood, propelled by the contrac- 
tions of the muscular walls of the second chamber, or ventricle, 
of the heart, and of the root (“ conus arteriosus,” Fig 6, b a) of 
the main trunk proceeding from the heart, next finds its way, 
by means of the branches of this main channel, into the leaves 
of that very compendious volume on respiration — the gills 
(“branchia”), and then discharges its ballast, carbonic acid, 
for a better cargo, oxygen.* 
Vein-roots, as many in number as the arterial branches 
which conveyed the impure blood to the gills, collect and 
carry with them the regenerated fluid to a kind of rendezvous 
at the base of the skull — the “ aortic circle” (“circulus cepha- 
licus ”) — from which the greater part flows on in a single main 
trunk, or aorta, to be distributed, through various branches, to 
various parts of the body. 
Having thus very briefly considered the means by which the 
blood is brought to the gills, there to exchange the old for 
that which is new, the impure for the pure, and the deadly 
for that which sustains life, we will next proceed to study more 
in detail the process by which water is admitted to the gills, 
and the way in which these organs first make the most, and 
afterwards rid themselves, of the streams which bathe their 
fringing folds. 
If a fish — for instance, a pike — be observed under fairly 
natural conditions, e.g. in the water of a large aquarium, it 
will be seen from time to time to make a kind of gulping, 
swallowing movement with its mouth. After the jaws have 
been opened, there is a slight, but appreciable, pause, after 
which a whitish fleshy fold or curtainf will be seen to descend 
from the interior, not the edge, of the upper jaw, a little 
* It must be borne in mind that the gill-laboratory of the fish does not 
rob, as by a sort of electrolysis, the water of that oxygen which is one of 
its two elementary constituents, but only of the free oxygen derived from 
the air held in solution by the water. Although the quantity of free oxygen 
present in water is much less, volume for volume, than that contained in 
air, seeing that the source of such oxygen is the air itself, held in solution 
in the water, this imprisoned air is, on the other hand, somewhat richer in 
oxygen than ordinary atmospheric air, for the reason that oxygen is twice as 
soluble as nitrogen in water. 
j- Cuvier’s description of this structure is so good that it is worth while 
to transcribe it here in full:— “II y a generalement en dedans de chaque 
machoire, derriere les dents anterieures, une espece de voile membraneux 
ou de valvule formee par un repli de la peau interieure et dirigee en arriere. 
