96 
Observations on the Natural Histmy 
primary trunk, beside its office of conveying blood to the gills, 
corresponds, in other respects, to the common carotid. 
The other or second branch of the primary trunk, is conduct- 
ed beneath the second or middle arch ; and having reached the 
place where the third arch joins with the second, it sends off an 
artery which runs along the margin of the third arch, and goes 
to form the third gill. The main portion of this second branch 
then proceeds along the middle arch ; and a little before reach- 
ing its posterior extremity, sends another artery for the forma- 
tion of the middle gill. After this, turning upward and inward, 
it goes for a short way towards the occiput ; and getting near 
the second vertebra, it bends backward and downward by the 
side of the spine ; towards the fourth vertebra, it meets, under 
the spine, with its fellow from the opposite side, and both then 
uniting, form together the aorta descendens , which is continued 
towards the tail. It is not necessary to follow the aorta through 
all the branches it gives off in its descent. We must not, how- 
ever, omit to mention, that this second or chief branch of the 
primary trunk, before it bends backward and downward, 
sends off three branches, and makes also an anastomosis with 
the common carotid. Of the three branches, the first is sent 
to the air-bladder, and to the ovaries in the female, and testicle 
in the male. The second is distributed to the parts about the 
temple ; and the third is the vertebral artery* which, after giving 
off some twigs to the occiput, enters the canal of the vertebrae, 
and descends towards the tail. This description of the circulat- 
ing system, is accurately represented in Plate VII. Fig. 3. 
Let us next follow the course of the branchial arteries, destin- 
ed, as we have seen, to form the gills. Immediately on quitting 
the branchial arches, they are continued out of the head ; and 
scarcely have they gone out, than they divide into many 
branches, which, in their turn, subdivide into others, and these, 
again, into minute ramifications, in such a manner, that the three 
gills resemble three little plantules, rooted to the sides of the oc- 
ciput, and furnished with many minute leaflets. The reason 
why the structure of the gills is so similar to that of a leaf, is, 
that the fine skin which lines the fauces, invests the arteries as 
they traverse the arches ; and, following them externally, clothes 
them as thev divide : but as they approach their last divisions, 
