FISHES. 321 



to its end, or posterior base, eight. Dr Broussonet, who 

 proposed these abbreviations of relative position, likewise 

 proposed to divide the body into as many regions as there 

 are measures equal to the length from the snout to the base 

 of the pectoral fin. Thus D 3 will indicate the dorsal fin 

 to be in the third region, while V. 3. 4. will intimate, that 

 the ventral fin is situate in the third and fourth regions *, 

 Tliis method presupposes a proportional increase in the di- 

 mensions of the different parts, during the growth of the 

 animal. 



The Atr-bag, as intimately connected ^vith the organs 

 of motion, here merits some notice. This organ is called by 

 some the Swimming-bladder, by others the Air-bladder. 

 It is the vesica natatoria of Willoughby, and the vesica 

 a'erea of Artedi. In this country it is called the sound. 

 When present) it is situate in the anterior part of the ab- 

 dominal cavity, and adheres to the spine. It is wanting in 

 the chondropterygii, and even in some of the osseous fishes, 

 as the flounder and mackrel. 



It is very different in shape, according to the species. 

 In the herring, and some other fishes, it is oblong, and 

 pointed at both ends. In the salmon it is obtuse at both 

 ends. In the burbot it is obtuse in the posterior end, and 

 bifid at its anterior extremity. In the carp it is divided 

 transversely, and in the silurus, longitudinally, into two 

 lobes. 



In general there is a duct (ductus pneumaticusj, by 

 means of which this air-bag communicates with the oesopha- 

 gus, or the stomach. In the sturgeon, there is a round 

 hole, nearly one inch in diameter, in the upper and back 

 part of the stomach, commvinicating with the air-bag. The 

 hole is surrounded by thin muscvilar fibres placed between 

 the membranes of the stomach and air-bag, which decussate 



• Phil. Trans. ]781, p. 442. 

 vol. II. X 



