41G THE MECHANICAL FUNCTIONS. 



that are wanted in the fish, for striking the water 

 laterally, with the broad vertical surface of the 

 tail. Processes of a similar form and appear- 

 ance, (F, F), and which impede any flexion 

 downwards, are generally also met with in 

 the lower surface of the spine, and more espe- 

 cially in the hinder portion of the column. These 

 are the inferior spinous processes, and, like the 

 superior, they also form an arch, through 

 which there passes the continuation of the 

 abdominal aorta, or great artery which proceeds 

 down the back. The number of vertebrae is 

 very various in different fishes : in some they 

 are multiplied exceedingly, as in the shark, 

 where there are more than two hundred. 



There are few parts of the structure of animals 

 that exhibit more remarkable instances of the 

 law of gradation than the spine of fishes, in 

 which we may trace a regular progress of deve- 

 lopement from the simplest and almost rudi- 

 mental condition in which it exists in the 

 Myxine and the Lamprey, to that of the most 

 perfect of the osseous tribes. Its condition, in 

 the former of these animals, presents a close 

 analogy with some structures that are met 

 with in the molluscous, and even in annulose 

 animals. So near is the resemblance of the 

 spinal column of the myxine, more especially, to 

 the annular condition of the frame-work of the 

 vermes, that doubts have often arisen in the 



