370 



THE MECHANICAL FUNCTIONS. 



which give it the lateral flexions that have been 

 just described. For this purpose all the important 

 viscera are placed forwards, and crowded towards 

 the head. No room is allowed for a neck ; and the 

 abdomen may be almost regarded as continuous 

 with the head, there being properly no intervening 

 thorax; for the respiratory organs are situated 

 rather beneath than behind the head. All this has 

 been done with a view to leave ample scope for the 

 prolonged expansion of the coccygeal vertebrae, 

 and of their muscles, which compose more than 

 half the bulk of the animal. 



Having seen how all impediments to the free 

 motion of the tail have been carefully removed, let 

 us next inquire into the mechanism by which mo- 

 bility has been given to that organ. The first 

 peculiarity we meet M-ith in the structure of the 

 spine of fishes is the mode in which the vertebrae 

 are connected together. The bodies of each verte- 

 bra, as may be seen in Figures 186 and 187, are 



// /I 11 I! _is^ 



187 



hollowed out, both before and behind, (considering 

 the spinal column as extended horizontally), so as 

 to form cup-like hollows ; by which means, where 

 the concave surfaces of two adjacent vertebrae are 

 applied to one another, a cavity, having the shape 



