FISHES. 51 



fins, situated more on the side, with their muscles pass through 

 a fissure in the anterior part of each of the lateral muscles, which 

 here divide into two bundles. By means of the lateral muscles 

 the spinal column is bent to the right or the left. The principal 

 means of progression for the fish consist in the lateral flexures, the 

 alternate relaxations and curvatures of the tail, or, in the case of 

 cylindrical fishes, as the eel, of the entire trunk 1 . The surface 

 which reacts upon the water may be greater or less in proportion 

 as the vertical fins (the dorsal, anal and caudal fins), of which the 

 rays are capable of separation from each other and of erection, are 

 more or less expanded. This is effected by muscles, two in number 

 on each side of the interspinal bones : one in front and one behind 

 the projecting line seen on each side of these bones. That in front 

 moves the ray forward and thus extends the fin ; that behind draws 

 the ray towards the back, or in the case of the anal fin towards the 

 belly, and thus depresses the fin. Above these muscles there 

 is situated on each side of every ray a superficial muscle inserted 

 into the skin ; these last, of which the fibres have an oblique direc- 

 tion as regards the large lateral muscles, move the fins laterally. 



The ventral and pectoral fins act as oars, and serve the fish 

 principally in directing its course and securing its position in the 

 water. In some fishes the pectoral fins are large enough to be able 

 to support the body in the air for a certain time (flying fishes, 

 Exocoetus, Dactylopterus). The pectoral fins can be moved from 

 or towards the body, can be expanded and moved up or down. 

 The forward and backward motion is dependent on the osseous belt 

 to which these fins are attached, and is very limited. These fins 

 are moved by muscles which are attached to the inner and outer 

 surface and inserted into the rays. Those of the inner surface 

 draw the pectoral fins nearer to the body (musculi adductores), those 

 I of the outer surface move them from it (musculi abductores). 

 [The ventral fins can draw their rays together or render them more 

 remote from each other, can move themselves perpendicularly 

 [downwards or horizontally outwards and inwards. 



The head is very slightly, if at all, moveable on the trunk ; the 

 jjaws, the palatine arch, the tongue-bone, the branchial arches, on the 



1 S. J. BRUGMANS pointed out that the stream of water issuing from the gill- 

 iperture on expiration, also assists the progressive motion of fishes. Verhandel. der 

 Klasse van het ffollandsche Instituut, i. bl. 185 217. Amsterd. 1812. 



42 



