MUTUAL CONNEXIONS OP VERTEBRA. 473 



instead of being supported upon foui fixed points, are buoyed 

 up in every part by a liquid of nearly the same density with 

 themselves. The extreme flexibility of the spine of Fishes, 

 enables them to propel their bodies by the movements of the 

 hinder portion and tail from side to side ; their members, or 

 pectoral and ventral fins (fig. 243), being but little used 

 except for influencing the direction of their motion. And 

 thus we see that in the lowest Vertebrate, as in the lower 

 Articulata (such as the Leech and Earth-worm), the propul- 

 sion of the body being accomplished by the movements of 

 the trunk itself, its skeleton (internal in the one case, external 

 in the other) is left in the soft condition which it has in all 

 at an early period : whilst in the higher classes of both series, 

 Birds and Insects for example, the extremities being so 

 developed, and being furnished with muscles so powerful 

 that the function of locomotion is entirely committed to them, 

 the skeleton of the body undergoes great consolidation, its 

 various pieces being so knit together as to make the trunk 

 almost immovable. 



630. This knitting-together is partly accomplished by 

 means of projections or processes from^the several vertebrae, 

 which are united to one another by muscles and ligaments. 

 Of these processes there are seven in Man from each vertebra. 

 One of these, termed the spinous process (b, fig. 225), projects 

 directly backwards ; and thus is formed the prominent ridge 

 on the back, in which the ends of these projections can be 

 distinguished. The spinous processes serve in Man to give 

 attachment to the muscles, by which the trunk and head are 

 kept erect ; in Animals whose spine is horizontal, they are 

 generally much longer, in order to give firm attachment to the 

 muscles and ligaments which support the head (fig. 229, vc, vd). 

 And in Fishes they are greatly prolonged (fig. 243), so as to in- 

 crease the surface by the stroke of which from side to side the 

 body is propelled through the water. On each side of the 

 vertebra is a process (c, fig. 225) which is called transverse; 

 this serves for the attachment of the ribs to the vertebra. 

 And lastly, from the upper and under side of each vertebra, 

 two articulating processes project, which lock against each 

 other in such a manner as to prevent the movements of the 

 vertebrse from being carried to an injurious extent. These 

 processes are peculiarly long in Birds, where they almost 



