252 ANATOMY OF VERTEBRATES. 



nating antagonistic contractions, the caudal moiety of the trunk, 

 producing that douljle lasli of the tail by which the fish darts 

 forwards with such velocity. When the lateral muscles are more 

 violently contracted, so as to bend the whole trunk, the recoil 

 may even raise and propel the fish some distance from its native 

 element : thus the salmon overleaps the roaring cataract which 

 opposes its migration to the shallow sources whither an irresistiljle 

 instinct impels it to the business of spawning ; and thus the 

 flying-fisli, in the extremity of danger, baffles its j^ursuer by 

 springing aloft, and prolongs its oblique course through the air 

 by the aid of its outspread pectorals. When the anterior por- 

 tions of the creat lateral masses act from the trunk as a fixed 

 point upon the head, they move it rapidly and forcibly from side 

 to side : in tliis way tlie Siluri deal severe blows with their out- 

 stretclied serrated pectoral spines ; thus the Percoid and Cottoid 

 Fishes strike with their opercular spines ; and so likewise may 

 the Saw-fish {Prlstis) and Sword-fisli (^X/phia.i) wield their for- 

 midal^Ie weapons, although their deadly cut or thrust is commonly 

 delivered with the whole impetus of the onward course, the head 

 being rigidly fixed upon the trunk. 



The supracarlnales, combining with the dorsal jiortions of the 

 myocommata, give tension to the region of the back, slightly 

 raise the tail, and depress the dorsal fins. Tlie infracarinales, in 

 combination witli the retractores pubis, tend to compress the 

 abdomen, to constrict the anus, and to depress the tail. 



The muscles of the pectoral fins, tliougli, compared with those 

 of the homologous members in higher Vertebrates, they are very 

 small, few, and simple, yet suffice for all tlie requisite movements 

 of the fins ; elevating, depressing, advancing, and aixain lavino- 

 them prone and flat, by an oblique stroke, upon tlie sides of the 

 body. The rays or digits of both pectorals and ventrals, as well 

 as those of the median fins, can be divaricated and approximated, 

 tlie intervening webs spread out or folded up, and the extent of 

 surface rer[ulred to react upon the ambient medium in each chano-e 

 and degree of motion, can be duly regulated at pleasure. 



In the ordinary forward movement the tail first Itends from the 

 vertebral axis, whicli is tlie axis of motion, fig. 159, f] (/, to a. 

 During this action the centre of gravity, r, slightly recedes. 

 From a the tall Is next forcibly bent by the muscles on the 

 opposite side, in tlie direction of the line a i. The force of the 

 action u])on the water in /i i is translated to the body in / a, 

 causing the centre of gravity, c, to move obliquely forward, in 

 tlie direcllon of r h, parallel to i a. The tail, continulno- its 



