14 Observations on the Natural History 



there this skin, instead of surrounding them individually, as it 

 had done in the larger branches, expands into a membrane, 

 which comprises between its two surfaces all the ultimate ramifi- 

 cations into which each lesser branch has spread. A good idea 

 of this foliated structure may be obtained, by inspecting Fig. 4. 

 Plate VII., in which a portion of one of the leaflets is represented, 

 and where the blood of the branchial artery, of a dark colour, is 

 seen moving round the margin of the leaflet. In its course, it con- 

 tinually passes off transversely through the net- work of vessels 

 that forms the expanded portion of the leaflet ; and, losing in 

 its progress its dark colout, and acquiring a florid hue, it is 

 finally collected from the several leaflets into the branchial vein 

 at the root of the gill *. 



Scarcely have these branchial veins reached the roots of the 

 gills, than they separate from their accompanying arteries, and 

 entering between the extreme points of the arches, proceed su- 

 perficially towards the top of the spine. The vein of the first 

 gill enters between the first and middle arches, and soon after 

 pours its blood into the first branch of the primary trunk, or 

 that named Common Carotid. The two other veins, on the 

 contrary, in re-entering the head, pass between the middle and 

 third arches : Afterwards, they unite into a single canal ; and 

 thus reunited, they deliver their blood into the second branch 

 of the primary trunk, a little before that vessel sends off^ its 

 branch to the air-bladder and sexual organs. This distribu- 

 tion of vessels conveying dark blood to the gills, and of those 

 which carry back florid blood from those organs, is repre- 

 sented in Fig. 3., as above. Of the subsequent distribution of 



* The authors remark, that the structure of these leaflets of the gills is visi- 

 ble only in dead protti, especially in those which, from having been placed in 

 spirits, have lost their transparency. In the living animal, while remaining in 

 water, they can be seen but with the greatest difficulty ; and then only when very 

 turgid with blood. The reason is, that the membranous expansions, between 

 which the ultimate ramifications of the arteries and veins are comprised, are so 

 exceedingly fine and transparent that an inexperienced eye is unable to perceive 

 them ; and an observer, in such circumstances, being unable to use a high magni- 

 fying power, and seeing only the vessels which bound the margins of the leaflets, 

 have supposed the gills to be formed rather like a dissected than entire leaf, or re- 

 sembling somewhat the horn of a stag. 



