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and Structure of the Proteus Anguinus. 13 



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 liave 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 they divide : but as they approach their last divisions, 



