MUSCULAR POWKR. 



127 



shorten the canal ; while that of the circular fibres, 

 by the yielding of the coats, and the partial re- 

 action of the contents of the vessel, has a tendency 



to extend it. The Ascidia, which is a species of 

 marine worm, is an example of an animal whose 

 skin contains a multitude both of straight and cir- 

 cular fibres, by which all its movements are readily 

 performed. Many instances occur in the cylin- 

 drical 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 external one has lately been discovered, which 

 is composed of oblique or spiral fibres, crossing one 

 another in opposite directions, and greatly facili- 

 tating the varied movements of the animal.* 



A variety of still more complicated arrangements 

 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, conse- 

 quently, unite the effects of both longitudinal and 

 circular fibres; and, when combined with either of 



Cams, Tabiilee Aiiat. Comp. fol. Tab, I. Fig. 6. 



