10G 



mose or exosmose ; and, as there has yet been no external opening 

 discovered, it seems difficult to comprehend what can become of the 

 air which is constantly being secreted by the gland. It would be 

 much more reasonable to suppose that this so called gland should be 

 subservient to the function of purification of the blood, or of the air, 

 than in that of secretion, and that the accomplishment of this purpose 

 may be the reverse of what it is in the lungs of Mammalia ; that is, 

 instead of the air being brought in contact with the blood, the blood 

 in the gland is brought in contact with the air. An arrangement of 

 parallel vessels, similar to those in the glandular bodies of the eel 

 and cod, have been found in the choroid gland of the eye of fishes, 

 and also on a large scale, forming two large glands, near v the liver of 

 the tunny. We can hardly imagine that these glands, so placed, are 

 for the secretion of air. When the circulation of fishes is taken into 

 account, when the power which impels the blood through the aorta 

 is merely the vis a tergo, it would not be difficult to suppose that 

 these glands are so arranged, that, instead of the blood of these par- 

 ticular organs going at once back to the gills to be purified, this neces- 

 sary change is accomplished by the minute division of the capillaries 

 of these glands. That the air-bladder does act as a float in some fish 

 may be true ; but if it be punctured, the fish is not deprived of the 

 power of raising itself in the water, as many have supposed, for 

 numerous experiments have been made, which are conclusive as to 

 this point. But the best of all evidence is to be derived from the 

 arrangement of the blood-vessels, which appear to have been over- 

 looked. On a careful comparison of the minute ramifications of the 

 capillary vessels in the air-bladder of the eel with those of other 

 injected tissues in my own possession, and also with those depicted in 

 works on microscopic anatomy, I find that the arrangement comes 

 nearest that of the lungs of reptiles, where a single vessel gives off or 

 divides into a great number of equal sized branches. In the toad, 

 frog, and snake, the only difference to be observed is, that the hexa- 

 gonal spaces between the capillaries are rather more uniform, and 

 much smaller in size, than in the eel ; but this may be accounted for, 

 in some measure, by the manner in which the object has been pre- 

 pared, as in drying, the membrane has been stretched in one way 

 more than another, so that the hexagons have given place to elongated 

 spaces, but the chief difference is on account of the interior not being 

 divided into cells, as in those of the Reptilia. In some air-bladders, 

 and even in the lower or posterior compartment of the air-bladder of 

 the eel, there is an arrangement of wavy vessels, very analogous to 



