SKELETON OF I ISHES. 371 



of a double cone, is formed by the junction of the 

 margins of these conical hollows. These cavities 

 are distinctly seen laid open in Fig. 188, which 

 represents a vertical section of three adjacent 

 vertebras of a cod. The edges that are in contact 

 are united all round by an elastic ligament, which 

 readily yields to the bending of the vertebras upon 

 one another 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 multi- 

 plied in the whole series, the effect which results 

 is considerable. The cavity itself is filled with 

 a gelatinous, but incompressible fluid substance, 

 which constitutes a spherical pivot for all the mo- 

 tions of the joint. 



This singular kind of articulation would appear 

 framed with a view to allow of motion in all direc- 

 tions. 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 which are wanted in the fish, for striking 

 the water laterally, with the broad vertical surface 

 of the tail. Processes of a similar form and ap- 

 pearance, (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 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 num- 



