SPINE OF THE VERTEBRATA. 



87 



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 the two spinous processes, ff? stand- 

 ing 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 verte- 

 brata. 



248. The design of this difference will immediately be- 

 come 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 

 vertebra? standing in a vertical position, when the fish 

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

 the spinal column upward and downward, is entirely 

 prevented, 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 

 backward and forward in the stern, it is plain that any 

 compound flexure of the spine would rather retard than 

 facilitate its progress. 



249. Locomotion of Fishes. The 

 manner in which fishes give themselves 

 progressive motion through the water 

 will be understood by Fig. 69. Suppose 

 that the tail is curved to the right, as / 

 shown in the figure, and in this situa-' 2 "' 

 tion, the muscles on the left side act 

 suddenly, so as to bring the fish into a 

 straight line, then the reaction 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 reaction, and the centre of 

 gravity c, would move in the direction 

 of c, 6, which is parallel to p, r. This 

 impulse is not destroyed by the next 

 flexure of the tail in the contrary 



