SKELETON OF FISHES. 289 



ther by the application of any force to one side of the spine, 

 and restores it to its former state, when the force has ceased 

 to act. The extent of motion in each joint is but small; but 

 being multiplied in the whole series, the resulting effect is 

 considerable. The cavity itself is filled with a gelatinous, 

 but incompressible fluid substance, which constitutes a sphe- 

 rical pivot for all the motions of the joint. 



This singular kind of articulation would appear framed 

 with a view to allow of motion in all directions. Here, 

 however, the motions are restricted by the extension of the 

 spinous processes (s, s, in the preceding figures,) which in 

 fishes are of great length; so that they effectually prevent all 

 flexions either upwards or downwards, and limit it to those 

 from side to side. It is precisely these latter kind of motions 

 *. that are wanted in the fish, for striking the water laterally, 

 with the broad vertical surface of the tail. Processes of a si- 

 milar form and appearance, f, f,) and which impede any 

 flexion downwards, are generally also met with in the lower 

 surface of the spine, and more especially in the hinder por- 

 tion of the column. These are the i7ifenor spinous pro- 

 cesses, 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 num- 

 ber 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 ex- 

 hibit more remarkable instances of the law of gradation than 

 the spine of fishes, in which we may trace a regular progress 

 of development from the simplest and almost rudimental 

 condition in which it exists in the M^xi?ie and the Lam- 

 prey, 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 mol- 

 luscous, and even in annulose animals. So near is the resem- 

 blance of the spinal column of the myxine, more especially, 

 to the annular condition of the frame-work of the vermes, 

 Vol. I. 37 



