VOL. LXvJ] PHILOSOPHICAL TRANSACTIONS. 66g 



the great organ from the small ; and the whole of their inner edges are fixed to 

 the middle partition formerly described, also to the air bladder, and 3 or 4 ter- 

 minate on that surface which incloses the muscles of the back. These septa are 

 at the greatest distance from one another at their exterior edges near the skin, 

 to which they are united ; and as they pass from the skin towards their inner 

 attachments, they approach one another. Sometimes we find 2 uniting into one. 

 On that side next the muscles of the back, they are hollow from edge to edge, 

 answering to the shape of those muscles ; but become less and less so towards 

 the middle of the organ ; and from that towards the lower part of the organ, 

 they become curved in the other direction. At the anterior part of the large 

 organ, where it is nearly of an equal breadth, they run pretty parallel to one 

 another, and also pretty straight ; but where the organ becomes narrower, it 

 may be observed in some places, that 2 join or unite into 1 ; especially where a 

 nerve passes across. The termination of this organ at the tail is so very small 

 that I could not determine whether it consisted of one septum or more. The 

 distances between these septa will differ in fishes of different sizes. In a fish of 

 2 feet 4 inches in length, I found them to be about ^y of an inch distant from one 

 another ; and the breadth of the whole organ, at the broadest part, about an 

 inch and a quarter, in which space were 34 septa. The small organ has the 

 same kind of septa, in length passing from end to end of the organ, and in 

 breadth passing quite across; they run somewhat serpentine, not exactly in 

 straight lines. Their outer edges terminate on the outer surface of the organ, 

 which is in contact with the inner surface of the external muscle of the fin, and 

 their inner edges are in contact with the centre muscles. They differ very much 

 in breadth from one another ; the broadest being equal to one side of the 

 triangle, and the narrowest scarcely broader than the point or edge. They are 

 pretty nearly at equal distances from one another ; but much nearer than 

 those of the large organ, being only about -^^ part of an inch asunder : but they 

 are at a greater distance from one another towards the tail, in proportion to the 

 increase of breadth of the organ. The organ is about half an inch in breadth, 

 and has 14 septa. These septa, in both organs, are very tender in consistence, 

 being easily torn. They appear to answer the same purpose with the columns in 

 the torpedo, making walls or hutments for the sub-divisions, and are to be con- 

 sidered as making so many distinct organs. These septa are intersected trans- 

 versely by very thin plates or membranes, whose breadth is the distance between 

 any 2 septa, and therefore of different breadths in different parts ; broadest at 

 the edge which is next to the skin ; narrowest at tliat next to the centre of the 

 body, or to the middle partition which divides the 2 organs from one another. 

 Their lengths are equal to the breadths of the septa, between which they are 

 situated. There is a regular series of them continued from one end of any 2 



