CETACEA. 335 



differ from fishes, and which has also an obvious relation to 

 their peculiar mode of breathing, is in the form of the tail, 

 which, instead of being compressed laterally, and inflected 

 from side to side, as in fishes, is flattened horizontally, and 

 strikes the water in a vertical direction, thereby giving the 

 body a powerful impulsion, either towards the surface, when 

 the animal is constrained to rise, or downwards, when, by 

 diving, it hastens to escape from danger. 



All the essential and permanent parts of the skeleton of 

 vertebrated animals, that is, the spinal column, and its im- 

 mediate dependencies, the skull, the caudal prolongation, 

 and the ribs, are found in that of the Cetacea. The thorax 

 is carried very much forwards, especially in the whale, and 

 the neck is so short as to be scarcely recognisable: for the 

 object of the conformation is here, as in that of the fish, to 

 allow free scope for the movements of the tail, and ample 

 space for the lodgement of its muscles. For the purpose of 

 giving greater power and more extensive attachment to these 

 muscles, the transverse processes of the dorsal and lumbar 

 vertebrae are expanded both in length and breadth, and, be- 

 ing situated horizontally, offer no impediment to the vertical 

 flexure of the spine. For the same reason, the ribs are con- 

 tinued in a line with the transverse processes, and articu- 

 lated with their extremities, thus giving still farther breadth 

 to the trunk. 



As there is a total absence of hinder extremities, so there 

 is no enlargement of any of the vertebrae corresponding to a 

 sacrum, and the caudal vertebrae are uninterrupted continua- 

 tions of those of the trunk. They develope, however, parts 

 which are met with only among fishes and reptiles, namely, 

 arches composed of inferior leaves* and spinous processes, 

 enclosing and giving protection to a large artery. Although 

 the bones of the legs do not exist, yet there are found, in the 

 hinder and lower part of the trunk, concealed in the flesh, 

 and quite detached from the spine, two small bones, appa- 



* These leaves being- formed of cartilag-e, are generally lost when tlie 

 bones are macerated for the purpose of preparing- the skeleton. 



