426 THE MECHANICAL FUNCTIONS. 



moving which are seen at c : o is the opercu- 

 lum, or flap, which covers the gills : and n, the 

 nasal cavities, or organs of smell. The form of 

 the body, and disposition of the skeleton, allow 

 of their being inserted immediately on the parts 

 which they are intended to approximate. Hence 

 the use of long tendinous chords is dispensed 

 with.* 



The actions of the muscles are easily under- 

 stood from the nature of their insertions. In 

 general, the direction of the fibres is in some 

 degree oblique, with reference to the motion 

 performed. Two series of muscles are provided 

 for the movements of the tail, which consist 

 almost exclusively of lateral flexion, the whole 

 spine in some degree participating in this motion. 

 These muscles occupy the upper and lower 

 portions of the trunk ; their limits being strongly 

 marked by a line running longitudinally the 

 whole length of the body on each side. The 

 inclination of their fibres is somewhat different 

 in each. The advantage in point of velocity of 

 action which results from this obliquity has al- 

 ready been pointed out. 



Those fins which are in pairs are capable of 

 four motions ; namely, those of flexion and ex- 



* Between the layers of flesh, however, there occur slender 

 semi-transparent tendons, which give attachment to a series of 

 short muscular fibres, passing nearly at right angles between the 

 surfaces of the adjoining plates. See Sir A. Carlisle's account 

 of this structure in the Philosophical Transactions for 1806. 



II 



