SPINE OF THE VERTEBRATA 



01 



Fig. 68. purpose of showing the peculiarity of this 

 part in the Fish, and which forms one of the 

 elements of Fig. 66, which is marked f. 



In the vertebrae of the Fishes, therefore, 

 we see two spinous processes /, f, standing 

 opposite to each other, the one "above and 

 the other below the body, while the trans- 

 verse processes are wanting. These are the 

 points of difference between the spines of 

 this class of animals, and those of the land 

 vertebrata. 



The design of this difference will immediately become 

 obvious, if we stop for a moment to inquire what sort of 

 motion in the spine, is best calculated to impel the fish 

 through the element where it lives. The spines of the 

 vertebrae, standing in a vertical position, when the fish is 

 in its usual posture, all vertical motions, or flexures of the 

 spinal column upwards and downwards, is entirely pre- 

 vented, the motions being limited to flexures from side 

 to side. Now since the fish moves through the water, 

 on the same principle that a boat is propelled by what is 

 called sculling, that is, by a single oar moved backwards 

 and forwards in the stern, it is plain that any compound 

 flexure of the spine would rather retard, than facilitate 

 its progress. 



Locomotion of Fishes. The manner 

 in which fishes give themselves pro- 

 gressive motion through the water, 

 will be understood by Fig. 69. Sup- 

 pose that the tail is curved to the right 

 as shown in the figure, and in this situ- 

 ation the muscles on the left side act 

 suddenly, so as to bring the fish into a 

 straight line, then the re-action of the 

 water against the motion of the tail in 

 the direction of r,p, would give the 

 whole body an impulse contrary to 

 that of the re-action, and the centre 

 of gravity c, would move in the direc- 

 tion of c, 5, which is parallel to p, r. 

 This impulse is not destroyed by the 

 next flexure of the tail in the contrary 



Fig. 69. 



