296 THE MECHANICAL FUNCTIONS. 



which covers the gills : and n, the nasal cavities, or organs 

 of smell. The form of the hody, and disposition of the skele- 

 ton, 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 understood from the 

 nature of their insertions. In general, the direction of the 

 fibres is, in some degree, oblique, with reference to the mo- 

 tion 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 inchnation of their fibres is somewhat diffe- 

 rent in each. The advantage in point of velocity of action 

 which results from this obhquity has already been pointed 

 out. 



Those fins which are in pairs are capable of four motions ; 

 namely, those of flexion and extension, and also those of ex- 

 panding and closing the rays ; for each of which motions ap- 

 propriate muscles are provided : and, indeed, each ray is fur- 

 nished with a distinct muscular apparatus for its separate mo- 

 tion ; and these smaller muscles regulate with great nicety all 

 the movements of the fins, expanding or closing them like a 

 fan, according as their action is to be strengthened or relaxed. 

 This feathering of the fin, as it may be called, takes place in 

 most fishes, and is particularly observable in the tail of the 

 Esox, or pike tribe. Each ray of these fins, indeed, is fur- 

 nished with a distinct muscular apparatus, for its separate 

 motion. 



Whatever analogy may exist in the structure of the fins 



* Between the layers of flesh, however, there occur slender semi-transpa- 

 rent tendons, which give attachment to a series of short muscular fibres pass- 

 ing nearly at right angles between the surfaces of the adjoining plates. 

 See Sir A. Carlisle's account of this structure in the Philosophical Transac- 

 tions for 1806. 



