288 



THE MECHANICAL FUNCTIONS. 



For this purpose, all the important viscera are placed for- 

 wards, 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 inter- 

 vening 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 com- 

 pose 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 mobility has been given to that 

 organ. The first peculiarity we meet with in the structure 

 of the spine of fishes is the mode in which the vertebras are 

 connected together. The bodies of each vertebra, as may 

 be seen in Figures 1S6 and 187, are hollowed out, both be- 



fore 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 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 vertebrae 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 vertebrae upon one ano- 



