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before us, three in number, running always quite parallel to each 

 other: the middle one of the three is much smaller than the two outer 

 ones, and is the artery, whilst the two larger trunks are the veins, (PI. 

 xii. fig. 3). In those fish in which 1 have succeeded in injecting 

 both arteries and veins, three trunks are seen ; but when only the 

 veins have been filled, then there are but the two outer ones, the small 

 artery between them being just perceptible, in the uninjected state. 

 When the ramification of these trunks takes place, each of the three 

 divides into two branches, which branches are nearly of the same di- 

 ameter as the original trunks, and they cross each other obliquely, 

 and produce an appearance not unlike the Roman letter Y ; this can 

 be explained better by a reference to PI. xii. fig. 3, than by any ver- 

 bal description. At the back part of the bladder which is attached 

 to the spine, there is another set of vessels of an arborescent figure, 

 and they run at right angles to the body of the fish ; these appear to 

 be connected more with the kidneys than with the air-bladder. 



The parallel arrangement of vessels appears to be constant in the 

 air-bladders of fish, there being but slight modifications of this plan 

 in all the species of fish which have been examined : in those fish in 

 which the bladder is double, as in the carp, tench, barbel, chub, roach 

 &c, this arrangement is found only in the posterior compartment, in 

 many fish they can be well examined, even without the aid of injec- 

 tion, for the vessels are, in most cases, filled with blood, and the con- 

 trast between them and the silvery lining membrane of the bladder 

 makes them the more evident. 



In the pike the air-bladder is a single one, but it has an aperture of 

 communication, or ductus pneumaticus, but no gland, as in the cod. 

 In this fish the distribution is very analogous to that which prevails 

 in the cod, but instead of having only three vessels, there are as many 

 as five or six running quite parallel to each other (PI. xii. fig. 5); and 

 when a branching off takes place, each of the three trunks nearest to 

 the side from which the vessels are to arise splits into two, and again 

 six parallel trunks are formed ; these again divide and subdivide to 

 their terminations. It is only here and there that we have a crossing 

 of the vessels as in the cod, as the branches arc given off first from 

 the right side, then from the left, and so on alternately. 



In the perch, which is remarkable from its having an air-bladder 

 without any ductus pneumaticus as the cod-fish before alluded to, we 

 have a glandular body not in one compact mass, but scattered about 

 the interior. The bladder itself is very thin, and when distended oc- 

 cupies a very considerable portion of the abdominal cavity, and the 



