138 THE MECHANICAL FUNCTIONS. 



which is a species of marine worm, is an example 

 of an animal whose skin contains a union of 

 straight and circular fibres, by Avhich all its 

 movements are readily performed. Many in- 

 stances occur in the cylindrical envelopes of 

 animals, of the combination of a third series of 

 fibres, passing obliquely, with those which have 

 transverse and longitudinal directions. In the 

 muscular skin of the Leech, for example, besides 

 two internal layers of longitudinal fibres, an ex- 

 ternal one has lately been discovered, which is 

 composed of oblique or spiral fibres, crossing 

 one another in opposite directions, and greatly 

 facilitating the varied movements of the animal.* 

 A variety of still more complicated arrange- 

 ments may be traced in the fibres of those 

 muscles which invest hollow sacs, or receptacles, 

 such as the stomach, (Fig. 51,) and the heart, 

 (Fig. 52). We find, in the substance of these 

 organs, sets of fibres, which pass in a spiral 

 direction, and which, consequently, unite the 

 effects of both longitudinal and circular fibres ; 

 and, when combined with either of these, they 

 serve to modify and regidate the actions of each 

 organ in a great variety of ways.f 



* Carus, Tabulse Anat. Comp. fol. Tab. I. Fig. 6. 



t The muscular fibres of the heart are disposed in two layers ; 

 each set passing in a spiral course from the basis, or broad part, 

 to the point or apex ; but the direction of the turns being dif- 

 ferent in each, the two layers cross or decussate, producing the 



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